Slide 2 / 158. Mole Calculations

Transcription

1 Slide 1 / 158

2 Slide 2 / 158 Mole Calculations

3 Slide 3 / 158 Table of Contents Click on the topic to go to that section Avogadro's Number Molar Mass Molar Volume Percent Composition Emperical Formula

4 Slide 4 / 158 Avogadro's Number Return to Table of Contents

5 Slide 5 / 158 Atomic Mass Recall an atom's atomic mass is equal to the number of protons plus the number of neutrons in the atom. Atomic Number or number of protons (Z) Average Atomic Mass in amu 6 C The unit for atomic mass is amu. Carbon-12 has 6 protons and 6 neutrons. One amu is equal to 1/12 the mass of carbon-12 or approximately the mass of one proton or neutron mass of 1 proton = 1 amu mass of 1 neutron = 1 amu

6 Slide 6 / 158 Atomic Mass What if you wanted to measure the mass of one atom in the laboratory? Would it be possible? A single atom has a very small mass. One carbon atom has a mass of about 2.0 x grams.

7 Slide 7 / 158 The Mole It takes a lot of atoms to give us enough material to directly measure in a lab. Hydrogen has a mass of 1 amu. How many atoms of hydrogen would be needed to make a 1 gram sample of hydrogen? 602,200,000,000,000,000,000,000 The amount 6.02x10 23 is called Avogadro's number or a mole. How big is a mole?

8 Slide 8 / 158 Calculating Avogadro's Number The atomic mass of one carbon atom is amu or 2x10-23 g. How many carbon atoms would it take to get grams of carbon? Givens: mass of 1 carbon atom = 2x10-23 g total mass of carbon atoms = g 2x10-23 g x? of atoms = g? of atoms = g 2x10-23 g? of atoms = 6.02 x atoms

9 Slide 9 / 158 Holy Mole-y! 1 mole of pennies could be distributed to all the currently-living people of the world so that they could spend a million dollars per hour every hour (day and night) for the rest of their lives! One mole of marbles would cover the entire Earth (oceans included) for a depth of two miles! If you were able to count at the rate of 1 million numbers a second, it would take about 20 billion years to count out one mole!

10 Slide 10 / 158 The Mole A mole is just a grouping of numbers...like dozen, ream, etc. A dozen means 12 of something. A mole means 6.02 x of something. Common Grouping Quantities 1 dozen = 12 1 gross =144 1 ream = mole = 6.02 x 10 23

11 Slide 11 / How many eggs are in two dozen eggs? A 12 B 24 C D 2 E 6.02 x 10 23

12 Slide 12 / How many eggs are in half a dozen eggs? A 12 B 24 C 6 D 0.5 E 6.02 x 10 23

13 Slide 13 / How many particles of sand are in 0.5 moles of sand? A 1.5 x B 3.01 x C 6.02 x D 1.2 x E 6.02 x mole = 6.02 x 10 23

14 Slide 14 / How many pieces of gold dust are in 2 moles of gold dust? A 1.5 x B 3.01 x C 6.02 x D 1.2 x E 6.02 x mole = 6.02 x 10 23

15 Slide 15 / How many dozen eggs are in a container of 6 eggs? A 4 B 0.5 C 1 D 2 E 6.02 x 10 23

16 Slide 16 / How many dozen eggs are in a container of 18 eggs? A 1.5 B 3 C 0.67 D 2 E 6.02 x 10 23

17 Slide 17 / 158 Dimensional Analysis How many eggs are in 47 dozen eggs? The number 47 makes this question a bit more difficult to calculate mentally. In chemistry, it is common practice to use equalities and dimensional analysis to solve problems. Equalities are two quantities that are identical in value but use different units. Examples of equalities include: 1 dozen = 12 1 mole = 6.02x meter = 1000 millimeters 1 hour = 60 minutes

18 Slide 18 / 158 Dimensional Analysis How many eggs are in 47 dozen eggs? To solve the above problem: 1. Choose the equality with the correct dimensions or units: 1 dozen eggs = 12 eggs 2. Turn the equality into a fraction. Since 1 dozen and 12 are equal quantities 1 dozen = 12 can also be written as a fraction: 1 dozen eggs 12 eggs 12 eggs = = = 1 1 dozen eggs

19 Slide 19 / 158 Dimensional Analysis How many eggs are in 47 dozen eggs? 3. Start with your given: 47 dozen Use dimensional analysis! Choose the fraction that has the appropriate dimensions or units to convert from your given dimensions to your unknown dimensions. unknown dimension 1 dozen 12 eggs is in the numerator or 12 eggs 1 dozen 4. Multiply your given by your chosen fraction and cancel out the units: 47 dozen x 12 eggs = 1 dozen 564 eggs

20 Slide 20 / The Milky Way Galaxy may have up to 400 billion (4 x stars). How many moles of stars does it have? A 1.5 x 10 9 B 6.7 x C 10 D 0.5 E 6.02 x mole of stars = 6.02 x stars 6.02 x stars 1 mole of stars or 1 mole of stars 6.02 x stars

21 Slide 21 / Mathematicians estimate Earth's beaches contain nearly 5.6 x grains of sand. How many moles of sand are on Earth's beaches? A 1.5 B 9.3 x 10-3 C 10.0 D E 6.02 x mole = 6.02 x 10 23

22 Slide 22 / 158 Measuring Matter with Moles The mole is the SI unit for measuring the amount of particles in a chemical substance. One mole (mol) of a substance is 6.02 x representative particles of that substance. 1 mole of Carbon

23 Slide 23 / 158 Measuring Matter with Moles How many moles of Gold are there in a sample containing 3.01 x atoms of Gold? 1 mol = 6.02 x10 23 atoms Given quantity: 3.01 x Au atoms Equalities: 1 mol Au 6.02 x10 23 Au atoms or 6.02 x10 23 Au atoms 1 mol Au Solve and cancel units: 3.01 x Au atoms x 1 mol Au 6.02 x10 23 Au atoms = 0.5 mol Au

24 Slide 24 / How many atoms of titanium are in a sample containing 0.5 mole of titanium? A 1.5 x B 3.01 x C 6.02 x D 1.2 x E 6.02 x mol = 6.02 x 10 23

25 Slide 25 / How many atoms of sodium are in a sample containing 2.0 moles of sodium? A 1.5 x B 3.01 x C 6.02 x D 1.2 x E 6.02 x mol = 6.02 x 10 23

26 Slide 26 / How many moles of potassium are in a sample containing 3.01 x atoms of potassium? A 1.0 B 2.0 C 0.5 D 0.75 E 6.02 x mol = 6.02 x 10 23

27 Slide 27 / How many moles of potassium are in a sample containing 1.2 x atoms of potassium? A B C D E 0.25 mol 0.50 mol 1.0 mol 2.0 mol 3.0 mol 1 mol = 6.02 x 10 23

28 Slide 28 / How many moles of tungsten atoms are there in a sample containing 1.8 x atoms of tungsten? A B C D E 0.33 mol 0.50 mol 1.0 mol 1.5 mol 3.0 mol 1 mol = 6.02 x 10 23

29 Slide 29 / How many moles of silver are there in a pure sample containing 1.5 x atoms of silver? A B C D E 0.10 mol 0.25 mol 0.50 mol 1.0 mol 1.5 mol 1 mol = 6.02 x 10 23

30 Slide 30 / How many atoms are there in 5.00 mol of hafnium? A B C D E 6.02 x10 23 atoms 1.20 x atoms 1.20 x atoms 3.43 x atoms 3.01 x atoms 1 mol = 6.02 x 10 23

31 Slide 31 / 158 Measuring Matter with Moles A mole of ANY substance contains Avogadro s number of representative particles, or 6.02 x representative particles. 1 mole of C atoms = 6.02 X atoms of C 1 mole of bicycles = 6.02 x bicycles The term representative particle refers to the species or types of particles in the substance For Example: atoms, molecules, formula units, ions

32 Slide 32 / 158 Measuring Matter with Moles molecule of H 2 O H O H 18.0 amu Avagadro's number of molecules (6.02 x10 23 ) laboratory sample size 1 mol H 2 O (18.0 g) In 1 mole of carbon there are 6.02 x carbon atoms. In 1 mole of water there are 6.02 x water molecules. In 1 mole of NaCl there are 6.02 x formula units.

33 Slide 33 / Formula units refer to particles of compounds and molecules refer to particles of compounds. A B C molecular/covalent, ionic ionic, molecular/covalent atoms, molecular D E atoms, ionic ionic, atomic

34 Slide 34 / How many molecules are there in 2.10 mol CO2? A 3.79 x B 3.49 x C 1.05 x D 2.53 x E 1.26 x mol CO 2 = 6.02 x CO 2 molecules

35 Slide 35 / How many moles of helium atoms are there in a pure sample containing 6.02 x atoms of helium? A 2.0 mol B C D E 4.0 mol 6.0 mol 10.0 mol 2.4 x mol 1 mol He = 6.02 x He atoms

36 Slide 36 / How many moles of NaCl are there in a pure sample containing 6.02 x formula units of sodium chloride, NaCl? A 1.0 mol B C D E 2.0 mol 4.0 mol 6.0 mol 6.02 x mol 1 mol NaCl = 6.02 x NaCl formula units

37 Slide 37 / How many formula units of Pb(NO 3 ) 2 are there in 0.5 mole of Pb(NO 3 ) 2? A B C D E 0.5 formula units 2.0 formula units 3.01 x formula units 1.2 x formula units 6.02 x formula units 1 mol Pb(NO 3 ) 2 = 6.02 x Pb(NO 3 ) 2 formula units

38 Slide 38 / 158 Real World Application Calcium deficiency can cause osteoporosis (weakening of the bones). The minimum amount of calcium in 1 ml of blood should be around 1.3 x atoms. A patient has her blood tested and the lab finds there are 3 x 10-5 moles of calcium in the blood. Is this patient at risk for osteoporosis? (6.02 x atoms/mole) x (3 x 10-5 moles) = 1.8 x atoms Ca slide for answer This exceeds the normal range so they are OK!

39 Slide 39 / 158 Review: Ionic and Molecular Compounds The total number of atoms or ions in a compound depends on electronegativity and bonding. Ionic Compounds NaCl 1 Na + ion 1 Cl - ion K 2 CrO 4 2 K + ions 1 CrO 4 2- ion Molecular Compounds 2 hydrogen atoms H 2 O C 6 H 6 1 oxygen atom Sn(OH) 2 1 Sn 2 + ion 2 OH - ions 6 carbon atoms 6 hydrogen atoms

40 Slide 40 / 158 Review: Ionic and Molecular Compounds Chemicals are composed of more than one molecule or formula unit. To indicate more than one molecule or formulat unit, add a coefficient in front of the compound. Example: six molecules of carbon dioxide = 6CO 2. How many atoms of carbon and oxygen are in 6CO 2? 6 atoms of carbon and 12 atoms of oxygen Move to reveal answer

41 Slide 41 / 158 Review: Ionic and Molecular Compounds Ionic Compounds - Fill in 3NaCl 3 Na + ions 3 Cl - ions 2K 2 CrO 4 K + 2- ions CrO 4 ions Molecular Compounds - Fill in 6H 2 O hydrogen atoms C 6 H 6 oxygen atom 4Sn(OH) 2 Sn 2 + ions OH - ions carbon atoms hydrogen atoms

42 O H H Slide 42 / 158 Converting Moles to Number of Particles molecule of H 2 O laboratory sample size 18.0 amu Avagadro's number of molecules (6.02 x10 23 ) 1 mol H 2 O (18.0 g) In one mole of water molecules there are: 2 moles of hydrogen atoms 2 x 6.02x10 23 atoms of hydrogen H 2 O 1 mole of oxygen atoms 1 x 6.02x10 23 atoms of oxygen

43 Slide 43 / 158 Converting Moles to Number of Particles In 3 moles of sodium chloride formula units there are: 3 moles of Na + ions 3NaCl 3 moles of Cl - ions

44 Slide 44 / How many hydrogen atoms are in six molecules of ethylene glycol, the major component in antifreeze? The formula for ethylene glycol is: HOCH 2 CH 2 OH. A B C D E 6 atoms of H 36 atoms of H 6 x 6.02 x atoms of H 36 x 6.02 x atoms of H 6.02 x atoms of H

45 Slide 45 / How many CO 3 2- ions are in one formula unit of CaCO 3? A B C D E 1 ion 3 ions 6 x 6.02 x ions 36 x 6.02 x ions 6.02 x ions

46 Slide 46 / How many K + ions are there in two formula units of potassium hydroxide, 2KOH? A B 1 K + ion 2 K + ions C 1 x 6.02 x10 23 ions of K + D 2 x 6.02 x ions of K + E 3.12 x ions of K +

47 Slide 47 / How many sulfide ions (S 2- ) are there in 2.0 moles of ammonium sulfide,(nh 4 ) 2 S? A B C D E 2.0 ions 1.2 x ions 2.4 x ions 6.02 x ions none *Use two equalities to solve: 1 mol (NH 4 ) 2 S = 6.02 x (NH 4 ) 2 S formula units 1 (NH 4 ) 2 S formula unit = 1 S 2- ion

48 Slide 48 / How many ammonium ions (NH4 + ) are there in 2.0 moles of ammonium sulfide, (NH 4 ) 2 S? A B C D E 2.0 ions 1.2 x ions 2.4 x ions 6.02 x ions 8.0 ions *Use two equalities to solve: 1 mol (NH 4 ) 2 S = 6.02 x (NH 4 ) 2 S formula units 1 (NH 4 ) 2 S formula unit = 2 NH 4 + ions

49 Slide 49 / How many moles of O are in 2.4 X molecules of SO 3?

50 Slide 50 / 158 Real World Application Hemoglobin is a protein that carries O 2 around your body. The formula for it is approximately C 2800 H 4800 N 3200 O 800 S 8 Fe 4. If a patient has 2 x atoms of Fe, how many moles of Hb would be present? moles of Fe: n = N/Na --> (2 x atoms)/(6.02 x atoms/n)= 3.3 x 10-7 moles Fe slide for answer moles of Hb: There are 1 Hb/4 Fe --> (3.3 x 10-7 moles Fe)/(4 moles of Fe/Hb) = 8.2 x 10-8 moles Hb

51 Slide 51 / 158 Molar Mass Return to Table of Contents

52 Slide 52 / 158 Mass of Compounds The total mass of a chemical compound can be calculated by using the masses on the Periodic Table. Example: Calculate the mass of the compound Magnesium Chloride (MgCl 2 ) Mg = 2Cl = Mass of MgCl amu (2) amu amu

53 Slide 53 / 158 Masses of Elements/Compounds Atomic mass Formula mass or Formula weight (FW) Molecular mass or Molecular weight used for elements (like F, V, etc..) only used for ionic compounds (like NaCl, MgO, etc..) only used for molecular compounds (like CO 2, H 2 O, etc..) only Units amu amu amu

54 Slide 54 / What is the formula weight of sodium bromide? A B C D E amu amu amu amu amu

55 Slide 55 / What is the formula weight of Pb(NO3 ) 2? A B C D E amu amu amu amu amu

56 Slide 56 / What is the molecular mass of 3H2 O 2? A B C D E 17 amu 34 amu 68 amu 102 amu 204 amu

57 Slide 57 / 158 Molar Mass (M) A mole represents the number of atoms it takes to convert from a single atomic mass in amu to the same mass in grams. The mass in grams of one mole of any substance is its molar mass (M). Each of the bars shown below equals one mole of a pure element. 1 mole of Copper = g 1 mole of Aluminum = g

58 Slide 58 / 158 Molar Mass (M) One mole of carbon, sulfur and silver are shown. 1 mol of Carbon atoms = 12.0 g 1 mol of sulfur atoms 32.0 g of S 1 mol of silver = g of Ag

59 Slide 59 / 158 How is the atomic mass of an element related to the molar mass? Average atomic mass of 1 atom of Kr = 83.8 amu. 1 mole (6.02 x atoms) of Kr = 83.8 grams. The atomic mass of an element expressed in grams is the mass of one mole or molar mass (M) of the element. 1 mole of Kr = 83.8 grams Molar mass of Kr = 83.8 grams or 83.8 g/mol 1 mol

60 Gram atomic mass Slide 60 / 158 Molar Mass The molar mass of an element or compound can be used to convert directly from masses in amu to masses in grams. used for elements (like F, V, etc..) only Units grams Gram formula mass or Formula weight (FW) Gram molecular mass or Molecular weight used for ionic compounds (like NaCl, MgO, etc..) only used for molecular compounds (like CO 2, H 2 O, etc..) only grams grams

61 Slide 61 / 158 Molar Mass To convert from moles to mass, or vice versa, use dimensional analysis. How many moles of Helium are in 8.0 grams of Helium? Molar mass of He = 4.0 g/mol Given quantity: 8.0 g He Equalities: 1 mol He 4.0 g He or 4.0 g He 1 mol He Solve and cancel units: 8.0 g He x 1 mol He 4.0 g He = 2 mol He

62 Slide 62 / How many moles are in a 64-gram sample of pure sulfur? *Molar mass (M) of S =? g/mol

63 Slide 63 / How many moles are in a 72-gram sample of pure magnesium? *Molar mass (M) of Mg =? g/mol

64 Slide 64 / What is the mass, in grams, of 2 moles of carbon?

65 Slide 65 / What is the mass, in grams, of 5 moles of iron?

66 Slide 66 / How many grams is 2.0 mol neon atoms?

67 Slide 67 / 158 Diatomic Molecules (Recall the seven diatomic molecules: HONClBrIF) The molar mass of these molecules will be twice their atomic mass. Examples: M of hydrogen gas: H 2 = (2 x 1) = 2 g/mol M of bromine liquid: Br 2 = (2 x 79.9) = g/mol M of fluorine molecules: F 2 = (2 x 19) = 38 g/mol

68 Slide 68 / How many grams is 1.0 mole of hydrogen molecules?

69 Slide 69 / How many grams is 0.50 mol of oxygen molecules?

70 Slide 70 / 158 Molar Mass of a Compound The molar mass of a compound is the sum of the molar masses of all the elements in the compound. To calculate the molar mass of a compound, find the number of grams of each element in one mole of the compound. Then add the masses of the elements in the compound g/mol + 3(16.000) g/mol = g/mol Molar mass of SO 3 = g/mol

71 Slide 71 / 158 Molar Mass of a Compound Note the different molar masses of these two compounds. 180 g of glucose (C 6 H 12 O 6 ) = 1 mole glucose 18 g of H 2 O = 1 mole H 2 O

72 Slide 72 / 158 [*] 37 What is the molar mass of K2 O, potassium oxide?

73 Slide 73 / What is the molar mass of CaCO3, calcium carbonate found in eggshells? A B C D 50 amu 50 grams/mol 100 amu 100 grams/mol

74 Slide 74 / What is the molar mass of carbon dioxide? A B C D 28 amu 28 grams/mol 44 amu 44 grams/mol

75 Slide 75 / What is the molar mass of ammonia, NH3? A B C D 17 amu 17 grams/mol 45 amu 45 grams/mol

76 Slide 76 / 158 Summary of Molar Mass Molar mass is the mass of 1 mole of a substance (measured in g/mol). The atomic mass will be the same number as the molar mass (measured in amu). The difference is that the atomic mass refers to only one representative particle and molar mass refers to one mole (6.02 x ) of representative particles.

77 Slide 77 / What is the mass of one formula unit of NaCl? A 17 amu B C D 17 grams/mol 58.5 amu 58.5 grams/mol

78 Slide 78 / What is the mass of one molecule of water? A B C D 18 amu 18 grams/mol 8 amu 8 grams/mol

79 Slide 79 / The chemical formula of aspirin is C9 H 8 O 4. What is the mass of moles of aspirin?

80 Slide 80 / The molar mass of oxygen (O2 ) is: A B C D E equal to the mass of one mole of oxygen atoms g/mol 32.0 g/mol none of the above equal to the mass of one oxygen atom.

81 Slide 81 / There are more moles of CO 2 in a 44 g sample of carbon dioxide gas than there are moles of helium atoms in a box containing 2 x atoms of He. True False

82 Slide 82 / 158 How many grams of iron are in a 68 gram sample of Fe 2 O 3? M of Fe 2 O 3 = 160 g/mol M of Fe = 56g/mol 69 g Fe 2 O 3 x 1 mol Fe 2 O 3 x 2 mol Fe x 56g Fe = 48 g Fe 160g Fe 2 O 3 1 mol Fe 2 O 3 1 mol Fe Move to see answer

83 Slide 83 / 158 Molar Volume Return to Table of Contents

84 Slide 84 / 158 The Mole and the Volume of a Gas The volume of a gas varies with temperature and pressure. But comparisons between gases can be made by designating a standard temperature and pressure (STP).

85 Slide 85 / 158 STP (Standard Temperature and Pressure) Standard Temperature (T) is considered 0 C (273 K), the temperature at which water freezes. Standard Pressure (P) is considered 1 atmosphere (101.3 kpa), or the pressure of the atmosphere at sea level. At STP, 1 mole of gas occupies a volume of 22.4 liters (L) V m = 22.4 L For the time being, we will treat all gases as if they are at STP.

86 Slide 86 / 158 The Mole and the Volume of a Gas The volume occupied by one mole of gas is called the molar volume and has the symbol, V m. It is the same for all gases. At STP; V m = 22.4 liters (L). 1 mole of gas = 6.02 x particles = 22.4 L Notice that this statement does not depend on the type of gas. It's true of all gases.

87 Slide 87 / 158 The Mole and the Volume of a Gas At STP; V m = 22.4 liters (L). This is true because in a gas the molecules are so far apart that they take up almost no space...the volume of a gas is mostly empty space, regardless of the type of gas. So when it comes to volume (at STP) all gases are created equal. 1 mole of He gas occupies mole of HCl gas occupies 22.4

88 Slide 88 / Which of the following must be true about 2 moles of H 2 gas vs. 2 moles of CO 2 gas at the same temperature and pressure? A Each sample will have the same density The same # of atoms will be present in each B sample C Each sample will have the same mass D Each sample will occupy the same volume E None of these are true

89 Slide 89 / 158 The Mole and the Volume of a Gas At STP, 1 mole or, 6.02 x representative particles, of any gas occupies a volume of 22.4 L. The quantity 22.4 L is called the molar volume of a gas. At STP, how many moles of CO 2 are there in 11.2 L of CO 2? 1 mole of gas = 22.4 L gas Given quantity: 11.2 L of CO 2 Equalities: 1 mol CO L CO 2 or 22.4 L CO 2 1 mol CO 2 Solve and cancel units: 11.2 L CO 2 x 1 mol CO L CO 2 = 0.5 mol CO 2

90 Slide 90 / How many moles are there in 44.8 liters (at STP) of fluorine gas? * At STP 1 mol = 22.4 L

91 Slide 91 / How many moles of atoms are there in 22.4 liters (at STP) of Xenon?

92 Slide 92 / What is the volume (in liters at STP) of 1.00 mole of sulfur dioxide?

93 Slide 93 / What is the volume (in liters at STP) of 2.50 moles of carbon monoxide?

94 Slide 94 / What is the volume (in liters at STP) of 4.00 moles of Nitrogen?

95 Slide 95 / 158 Real World Application An inflated airbag requires 60 L of nitrogen gas (N 2 ) at STP in order to protect the occupant of the vehicle. How many moles of nitrogen gas would need to be created in 40 milliseconds to inflate the airbag?

96 Slide 96 / 158 Mole Calculations Sometimes multiple steps must be used in mole calculations. To solve these problems use dimensional analysis and the appropriate mole conversions. Here's an example: How many atoms of carbon are present in a 24 g sample of pure carbon? To solve convert from: grams moles atoms

97 Slide 97 / 158 Mole Calculations How many atoms of carbon are present in a 24 g sample? Start with your given: 24 g C Choose the correct equality to convert from grams to moles: M of C = 12g/mol Choose the correct equality to convert from moles to atoms: 1 mole = 6.02 x Solve and cancel out units: 1 mol C 12 g C 6.02 x atoms C or or 12 g C 1 mol C 1 mol C 1 mol C 6.02 x atoms C 24 g C x 1 mol C x 6.02 x atoms C 12 g C 1 mol C = 1.2 X 1024 atoms C

98 Slide 98 / 158 Mole Road Map Particles: atoms, ions, molecules, etc. 1 mol = 6.02 x particles Molar mass (M) 1 mol = grams found using the Periodic Table moles Molar Volume (V M ) 1 mol = 22.4 L m (mass) V (Volume)

99 Slide 99 / How many molecules are there in 44.8 liters (at STP) of oxygen gas? 1 mol O 2 molecules = 6.02 x O 2 molecules 1 mol O 2 gas = 22.4 L O 2

100 Slide 100 / How many atoms are there in 11.2 liters (at STP) of molecular oxygen ( oxygen gas)?

101 Slide 101 / What is the mass of 44.8 liters (at STP) of molecular oxygen?

102 Slide 102 / What is the volume (at STP) of 3.0 x molecules of fluorine?

103 Slide 103 / What is the volume (at STP) of 240 g of nitrogen gas?

104 Slide 104 / How many atoms are present in 30 g of boron? M of Boron= 10.8 g/mol or 1 mol = 10.8 g 1 mol B = 6.02 x atoms B

105 Slide 105 / What is the mass of a pure sample of lead which contains 3.0 x atoms?

106 Slide 106 / How many moles are there in 11.2 liters (at STP) of chlorine?

107 Slide 107 / What volume will 6.0x10 24 molecules of oxygen gas occupy at STP?

108 Slide 108 / How many molecules are in a 32g sample of SO2?

109 Slide 109 / Determine the volume occupied by 216g of N2 O 5 at STP?

110 Slide 110 / What volume will 5.0x10 24 atoms of krypton occupy at STP?

111 Slide 111 / What is the mass of 224 L of hexene (C6 H 12 ) at STP?

112 Slide 112 / What is the volume occupied by 6.0x10 23 atoms of Hydrogen at STP?

113 Slide 113 / How many atoms of C are in a 30 milligram sample of calcium cyanide. (Hint: you must remember that milli means 1/1000 and you must first write the proper formula for calcium cyanide)

114 Slide 114 / What is the mass of 44.8L of Argon gas at STP?

115 Slide 115 / 158 Percent Composition Return to Table of Contents

116 Slide 116 / 158 Identify an Unknown Substance We have been able to calculate the molar mass of a material if we know its formula already. However, what if we encounter an unidentified substance? Using the tools we already have, we are able to determine the composition of a substance if we have certain pieces of information.

117 Slide 117 / 158 Percent Composition One such piece of information is the percent composition, or the percentage of a compound's mass, made up by its various elements. Potassium chromate, K 2 CrO 4 Potassium dichromate, K 2 Cr 2 O 7

118 Slide 118 / 158 Percent Composition The percent by mass of an element in a compound is the number of grams of the element divided by the mass in grams of the compound, multiplied by 100%. % element = (number of atoms)(atomic weight) (FW of the compound) x 100

119 Slide 119 / 158 Percent Composition of Ethane The percent composition of carbon and hydrogen in ethane, (C 2 H 6 ) is %C = (2)(12.0 amu) 30.0 amu x 100 = 80.0% %H = (6)(1.0 amu) 30.0 amu x 100 = 20.0% Since ethane is made of only Carbon and Hydrogen, if the percent composition of Carbon was 80%, the remaining percent must be Hydrogen. Another method to calculate the %H is: %H = 100% - %C %H = 100% - 80% %H = 20%

120 Slide 120 / 158 Percent Composition of Ethane The percent composition of ethane, (C 2 H 6 ) is %H = 20% and %C = 80% Note that even though there are more ATOMS of hydrogen in ethane, there is a much less percentage of mass in the compound than that of carbon. This is because 1 carbon atom (12 amu) is much more massive than 1 hydrogen atom (1 amu). Therefore, carbon accounts for a much greater percentage of the mass of ethane than hydrogen does.

121 Slide 121 / In water (H2 O), which element do you think accounts for more mass? A Hydrogen B Oxygen C Hydrogen and Oxygen account for the same percent of mass D Carbon

122 Slide 122 / What is the mass percentage of oxygen in water?

123 Slide 123 / What is the percent by mass of carbon in acetone, C 3 H 6 O?

124 Slide 124 / What is the percent of Ba in Ba(NO3 ) 2

125 Slide 125 / Iron (II) oxide has a smaller % of iron by mass than iron (III) oxide. True False

126 Slide 126 / 158 Real World Application Aluminum is used in soda cans, aircraft frames, and automobile engines. Worldwide aluminum demand is increasing. Aluminum is isolated from an ore called bauxite which is roughly 12% aluminum by mass. How much bauxite ore (in grams) must be mined to provide enough aluminum to make a car engine requiring 100 kg of aluminum? bauxite ore Al block engine move for answer 100 kg x (1000 g/kg) x (100 g of ore/12 gram of Al) = 830,000 g Al

127 Slide 127 / A 2.00 sample of a compound containing only potassium and oxygen is heated. The oxygen gas leaves and the resulting mass of the potassium is 1.66 grams. What is the % by mass of oxygen in the compound? A 17% B 70% C 30% D 45% E Cannot be determined from the information

128 Slide 128 / What is the % by mass of water in the CaSO 4 *2H 2 O crystal? A 10% B 21% C 19% D 50% E 75%

129 Slide 129 / 158 Empirical Formula Return to Table of Contents

130 Slide 130 / 158 Calculating Empirical Formulas Now that we are able to calculate the percent of a compound by mass of each element, we can begin to identify unknown substances. First, we must identify the ratio of the number of moles of each element in the substance This formula, based on whole-number ratio, is called an empirical formula.

131 Slide 131 / 158 Calculating Empirical Formulas Empirical formula: The formula that indicates the molar ratio of elements present in a molecular compound reduced to the least common denominator. For instance, the empirical formula for benzene (C 6 H 6 ) is CH. Mass % elements Assume 100g sample Grams of each element use molar mass Moles of each element Calculate mole ratio Empirical formula

132 Slide 132 / 158 Calculating Empirical Formulas Empirical formula: There is only one empirical formula for a substance, but two different substances can have the same empirical formula. That is because an empirical formula is always based on the ratio of the elements given in their lowest common denominator. Substance Hydrogen peroxide Hydroxide Example: Molecular Formula H 2 O 2 OH Emperical Formula OH OH

133 Slide 133 / The empirical formula for C6 H 4 (NO 3 ) 2 is C 3 H 2 N 2 O 3 True False

134 Slide 134 / Which of the following has an empirical formula that is the same as its molecular formula? A B C D 1 and 3 only 1, 2 and 4 only 1 and 4 only all of them (1) NH 4 Cl (2) (NH 4 ) 2 CO 3 (3) CH 2 Cl 2 (4) CHCl 2 Br

135 Slide 135 / 158 Calculating Empirical Formulas Mass % elements The compound para-aminobenzoic acid (listed as PABA in bottles of sunscreen) is composed of the following elements (by mass): carbon (61.31%), hydrogen (5.14%), nitrogen (10.21%), and oxygen (23.33%). Find the empirical formula of PABA.

136 Slide 136 / 158 Calculating Empirical Formulas Mass % elements Assume 100g sample Grams of each element Step 1: convert the mass percentages to mole amounts in a hypothetical 100 g sample. Assume a g amount of PABA to represent the mass of each element: C: 61.31% x 100g = g H: 5.14 % x 100g = 5.14 g N: 10.21% x 100g = g O: % x 100g = g

137 Slide 137 / 158 Calculating Empirical Formulas Mass % elements Assume 100g sample Grams of each element use molar mass Moles of each element Step 2: Now that we know the representative mass, we can use the molar mass of each element to calculate the number of moles that would be present. 1 mol C: g x g = mol 1 mol H: 5.14 g x 1.01 g = 5.09 mol 1 mol N: g x = mol g O: g x 1 mol = mol g

138 Slide 138 / 158 Calculating Empirical Formulas Mass % elements Assume 100g sample Grams of each element use molar mass Moles of each element Calculate mole ratio Empirical formula Step 3: Calculate the mole ratio by dividing each mole value by the smallest number of moles. In this case, by that of nitrogen. Divide them all by C: mol mol mol = # 7 H: 5.09 mol N: mol O: mol 5.09 mol mol mol mol mol mol = # 7 = 1.00 = 1 = # 2 The empirical formula for PABA is C 7 H 7 NO 2

139 Slide 139 / 158 Calculating Empirical Formulas 1,6 - diaminohexane is used in making nylon. It is 62.1% C, 13.8% H and 24.1% N. What is the empirical formula? C: 62.1 g x 1 mol = 5.17 mol = 5.17 mol = 3.01 # g 1.72 mol H: 13.8 g x 1 mol = 13.8 mol = 13.8 mol = 7.96 # g 1.72 mol N: 24.1 g x 1 mol = 1.72 mol = 1.72 mol = 1.00 = g 1.72 mol The empirical formula is C 3 H 8 N

140 Slide 140 / What is the empirical formula for a compound with the following percent composition? A C H 4 B C 2 H 5 C C 3 H 6 D C 3 H 8 C: 74.9 % H: 25.1 %

141 Slide 141 / What is the empirical formula for a compound with the following percent composition? A C 2 H O B C H 2 O C C H O 2 C: 40.0 % H: 6.7 % O: 53.0 % D C 2 H 3 O 2

142 Slide 142 / 158 Special Cases The molar ratio works very well for most compounds. However, in some cases the math leaves us without whole numbers: For Example: iron (?) oxide Mass % elements Assume 100g sample Grams of each element use molar mass Moles of each element Calculate mole ratio Fe: 69.92% -> 69.92g = 1.25 mol = g/mol 1.25 mol O: 30.08% -> 30.08g = 1.88 mol = g/mol 1.25 mol FeO 1.5

143 Slide 143 / 158 Special Cases In these cases, it is necessary to multiply the results by an integer to ensure the ratio is a whole number ratio... x 2 = 2 x 2 = 3 The empirical formula is Fe 2 O 3 The name of the compound is iron (III) oxide

144 Slide 144 / What is the empirical formula for a compound with the following percent composition? A V O 1.67 B V 2 O 5 V: % O: % C V 3 O 5 D V 3 O 8

145 Slide 145 / A 4.68 gram sample of a sulfur oxide is heated releasing oxygen gas and leaving behind solid sulfur. If the mass of the sulfur left behind was 2.34 grams, what must be the empirical formula of the compound? A SO B SO 2 C S 2 O D S 2 O 3 E SO 3

146 Slide 146 / 158 Hydrated Crystals Many ionic compounds have a fixed quantity of water molecules bound within their structure. CuSO 4 * 5H 2 O The mole ratio of water to the dry CuSO 4 crystal is 5:1

147 Slide 147 / 158 Finding the formula of a hydrate One can find the formula of a hydrate by heating the crystal to remove the water. Then find the mole ratio between the dry crystal and the water just as you would when finding an empirical formula. CuSO 4 *? H 2 O HEAT g CuSO 4 --> n CuSO 4 g H 2 O --> n H 2 O Find mole ratio!

148 Slide 148 / 158 Finding the formula of a hydrate After heating, a hydrate of MgSO 4 was found to be roughly 51.3% water by mass. What is the formula of the hydrate? Step 1: Find the % of the crystal and water and express in grams % water = 48.7 % MgSO 4 = 48.7 g MgSO 4 & 51.3 g of water Step 2: Convert to moles 48.7 g MgSO 4 = mol MgSO g H 2 O = 2.85 mol H 2 O 120 g/mol 18 g/mol

149 Slide 149 / 158 Finding the formula of a hydrate Step 3: Find the mole ratio of the water to the dry crystal mol MgSO 4 = mol H 2 O = mol mol...the formula is MgSO 4 *7H 2 O

150 Slide 150 / When a 2.4 g sample of a hydrated crystal of BaCl 2 is heated, the dry anhydrous crystal has a mass of 2.08 grams after heating. What is the formula of the hydrate? A BaCl 2 *H 2 O B BaCl 2 *2H 2 O C BaCl 2 *3H 2 O D BaCl 2 *4H 2 O E BaCl 2 *5H 2 O

151 Slide 151 / 158 Molecular Formulas A molecular formula indicates the number of atoms of each element present in the molecule. The molecular formula of a compound is either the same as its experimentally determined empirical formula, or it is a simple whole-number multiple of its empirical formula

152 Slide 152 / 158 Molecular Formulas Formula Name Type of Formula Molar Mass CH Empirical 13 C 2 H 2 Ethyne Molecular (13x2 =) 26 C 6 H 6 Benzene Molecular (13x6 =) 78 CH 2 O Methanal Empirical & Molecular 30 C 2 H 4 O 2 Ethanoic Acid Molecular (30x2 =) 60 C 6 H 12 O 6 Glucose Molecular (30x6 =) 180

153 Slide 153 / 158 Molecular Formulas To find the actual molecular formula you need one other piece of information...the molecular weight (mass) of the molecule. If you know the ratio of the elements in a molecule and you know the total mass of the molecule then you can determine the molecular formula.

154 Slide 154 / 158 Molecular Formulas What is the molecular formula of a compound with an empirical formula of CH 2 O that has a mass of 180 g/mol? To find the molecular formula: Step 1: Determine the molar mass of the empirical formula. Molar mass of CH 2 O = 30 g/mol Step 2: Divide the molecular mass by the empirical mass Molecular mass = 180 g/mol = 6 Empirical mass 30 g/mol Step 3: Multiply the Empirical Formula by the resulting integer 6 x CH 2 O = C 6 H 12 O 6

155 Slide 155 / The molecular mass of Benzene is 78. If the empirical formula of benzene is CH, what is its molecular formula? A C 2 H 2 B CH C C 6 H 6 D C 2 H 4

156 Slide 156 / 158 Molecular Formulas Maleic acid is an organic compound composed of % carbon, 3.47% hydrogen, and the rest is oxygen. It has a molecular mass of 116 g/mole. Determine the empirical formula for Maleic acid. Then determine the molecular formula for Maleic acid.

157 Slide 157 / 158 Real World Application Clenbuterol is a steroid drug that is illegally used in cattle and in sports like cycling to help the cattle or athlete lose fat and gain lean muscle mass. It can be detected by mass spectroscopy. What is the empirical and molecular formula for clenbuterol if when a 10.0 g sample is combusted in air, it is found it contains 4.60 g of carbon, g of H, 0.51 g of oxygen with the rest being chlorine? The molecular weight is 313 g/mol.

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