Chemistry Chapter 3. Stoichiometry. (three sections for this chapter)

Transcription

1 Chemistry Chapter 3 Stoichiometry (three sections for this chapter)

2 Chemistry Chapter 3 Stoichiometry Section Average Atomic Mass The Mole Molar Mass

3 Average Atomic Mass Average mass of objects is required to count the objects by weighing For purposes of counting, objects behave as though they are all identical Samples of matter can contain huge numbers of atoms Number of atoms in a sample can be determined by finding its mass

4 Modern System of Atomic Masses Instituted in 1961 Standard - 12 C 12 C is assigned a mass of exactly 12 atomic mass units (u) Masses of all other atoms are given relative to this standard

5 Mass Spectrometer Helps to accurately compare the masses of atoms

6 Mass Spectrometer (continued 1) Atoms or molecules are passed into a beam of highspeed electrons Electrons are knocked off the atoms or molecules being analyzed and are changed into positive ions An applied electric field then accelerates these ions into a magnetic field

7 Mass Spectrometer (continued 2) An interaction with the applied magnetic field occurs as an accelerating ion produces its own magnetic field Tends to change the path of the ion Amount of path deflection of an ion depends on its mass The most massive ions are deflected the smallest amount Comparison of the positions where the ions hit the detector plate gives very accurate values of their relative masses

8 Uses of Mass Spectrometer Helps determine accurate mass values for individual atoms Determines the isotopic composition of natural elements

9 Question: If mass numbers are protons 1, neutrons 1 and electrons 0, how can there be decimals for the atomic masses? Average Atomic Mass the weighted average of the atomic masses of the naturally occurring isotopes of an element need average mass of the atoms atoms behave as though they were all identical

10 Average Atomic Mass Example: If you have 46 marbles that are 3.75g and 54 marbles that are 4.25g, what is the average mass of a marble? (.46 x 3.75g) + (.54 x 4.25g) 1.73g g 4.03g

11 Average Atomic Mass Example 2: If you have 7 marbles that are 1.50g and 13 marbles that are 1.80g, what is the average mass of a marble? (.35 x 1.50g) + (.65 x 1.80g) 0.525g g 1.70g

12 Average Atomic Mass Elements occur in nature as mixtures of isotopes For example, Carbon: 98.89% 12 C 1.11% 13 C < 0.01% 14 C

13 Average Atomic Mass for Carbon 98.89% of 12 amu % of amu % of amu = (0.9889)(12 amu) + (0.0111)( amu) + + (0.0001)( amu) = amu amu amu = amu amu

14 Average Atomic Mass for Carbon Even though natural carbon does not contain a single atom with mass 12.01, for calculation purposes, we can consider carbon to be composed of only one type of atom with a mass of This enables us to count atoms of natural carbon by weighing a sample of carbon

15 Question: What is the mass of one oxygen-16 atom? kg Relative Atomic Mass 2.679x10-26 kg Because masses are so small, must use atomic mass unit (amu) or (u)» the gram is just too small to use for atoms» 1 amu is equal to exactly 1/12 the mass of a C-12 atom» 12 C is the standard set for the periodic table» 12 C is assigned a mass of exactly 12 atomic mass units

16 Exercise An element consists of 62.60% of an isotope with mass amu and 37.40% of an isotope with mass amu. Calculate the average atomic mass and identify the element amu Rhenium (Re)

17 Exercise An element consists of: 1.40% of an isotope with mass amu 24.10% of an isotope with mass amu 22.10% of an isotope with mass amu 52.40% of an isotope with mass amu Calculate the average atomic mass, and identify the element Mass = u The element is lead (Pb) Copyright Cengage Learning. All rights reserved 17

18 Exercise When a sample of natural copper is vaporized and injected into a mass spectrometer, the results shown in the graph are obtained Use these data to compute the average mass of natural copper Mass values for 63 Cu and 65 Cu are 62.93u and 64.93u, respectively

19 Exercise The atomic number of Indium is 49 and its atomic mass 114.8g. Naturally occurring indium contains a mixture of indium-112 and indium-115, respectively, in an atomic ratio of approximately: A. 6:94 B. 25:75 C. 50:50 D. 75:25 E. 94:6

20 Exercise Who wants to BET????? Boron exists in nature in two main isotopic forms: B-10 & B-11. What is the natural abundance of B-11?

21 The Mole What is a mole?

22 The Mole What is a dozen? Can you have a dozen cookies? Can you have a dozen pencils? If you have a dozen cars, how many do you have? What is a dozen?

23 The Mole A counting number/unit (like a dozen) Also called Avagadro s Number (N A ) This number is named in honor of Amedeo Avagadro ( ) who studied quantities of gases and discovered that no matter which gas he studied, there were the same number of molecules present 6.022x ,200,000,000,000,000,000, billion trillion 1 mol = 6.022x10 23

24 The Mole 1 dozen cookies = 12 cookies 1 mole of cookies = X cookies 1 dozen cars = 12 cars 1 mole of cars = X cars 1 dozen Al atoms = 12 Al atoms 1 mole of Al atoms = X atoms Are the numbers the same for the examples? Are the masses the same for the examples? The NUMBER is always the same, but the MASS is very different!

25 Molar Mass The number equal to the number of carbon atoms in exactly 12 grams of pure 12 C. 1 mole of anything = x units of that thing (Avogadro s number) 1 mole C = x C atoms = 12.01g C

26 Molar Mass The mass of 1 mole of an element or compound Atomic mass tells the atomic mass units per atom (amu) Molar mass tells the mass in grams per mole (g/mol) Guidelines to Calculate Molar Mass List each element in the compound How many atoms are there of each element Look up the mass of each element Multiply the amount of atoms by the mass Add the masses to find the Molar Mass for the compound

27 Molar Mass Moles Atoms Mass 1 mol He 6.022x10 23 atoms He 4.00g He 1 mol C 6.022x10 23 atoms C 12.01g C 1 mol Ag 1 mole H 2 O 6.022x10 23 atoms Ag g Ag 6.022x10 23 molecules H 2 O 18.02g H 2 O H 2 x 1.01 = 2.02 O 1 x = = 18.02g H 2 0

28 Molar Mass Mass in grams of one mole of the substance: Molar Mass of N 2 = g/mol Molar Mass of CO 2 = g/mol Molar Mass of Ba(NO 3 ) 2 = g/mol Molar Mass of CuSO 4 2H 2 0 = g/mol

29 Concept Check Calculate the number of iron atoms in a 4.48 mole sample of iron Fe atoms

30 Concept Check Which of the following is closest to the average mass of one atom of copper? a) g b) g c) 29 g d) g e) x g

31 Concept Check Calculate the number of copper atoms in a g sample of copper Cu atoms

32 Concept Check How many atoms are there in a 50.0g sample of gold? 1.53x10 23 atoms Au

33 Concept Check How many moles are there in a 50.0g sample of gold? mols Au

34 Concept Check Which of the following g samples contains the greatest number of atoms? Magnesium Zinc Silver

35 Concept Check Rank the following according to number of atoms (greatest to least): A) g of silver B) 70.0 g of zinc C) 21.0 g of magnesium B A C

36 Mass Percent of an Element mass of element in compound mass % = 100% mass of compound For Carbon Dioxide: C 1 x = O 2 x = = (CO 2 ) x100 = x100 = 27.29% C 72.71% O

37 Mass Percent of an Element What is the mass percent for (NH 4 ) 2 SO 4? (NH 4 ) 2 SO N 2 x = H 8 x 1.01 = 8.08 S 1 x = O 4 x = = x100 = x100 = x100 = x100 = 21.2% N 6.1% H 24.2% S 48.4% O

38 Empirical Formula consists of the symbols for the elements combined in a compound, with subscripts showing the smallest whole-number mole ratio of the different atoms in the compound Guidelines to calculate Empirical Formula Convert each element into moles If given in percent, assume to have 100g Make it look like a formula Divide by the smallest molar number If needed, make it a whole number Note: Sig figs do not matter so keep more than less numbers

39 Concept Check If a compound has 94g oxygen and 6g hydrogen, what is its empirical formula? OH

40 Concept Check If a compound has 119.8g carbon and 20.3g hydrogen, what is its empirical formula? CH 2

41 Concept Check Who wants to BET????? The composition of adipic acid is 49.3% C, 6.9% H, and 43.8% O (by mass). What is the empirical formula? C 3 H 5 O 2

42 Molecular Formula the actual formula for a molecular compound it has not been simplified X (empirical formula) = molecular formula X (empirical formula mass) = molecular formula mass Guidelines for solving Molecular Formula Need to have the Empirical Formula determined Solve for X to find the multiplier Multiply the empirical formula by the multiplier to get the correct molecular formula

43 Molecular Formula (book explanation) Empirical formula = CH Molecular formula = (empirical formula) n [n = integer] X (empirical formula) = molecular formula Molecular formula = C 6 H 6 = (CH) 6

44 Concept Check The empirical formula of a compound is known to be N 2 O 3. It was found experimentally that its molar mass is g/mol. What is the compounds molecular formula? N 6 O 9

45 Concept Check The composition of adipic acid is 49.3% C, 6.9% H, and 43.8% O (by mass). The molar mass of the compound is about 146 g/mol. What is the molecular formula? C 6 H 10 O 4

46 Concept Check Who wants to BET????? Caffeine, a stimulant found in coffee (1.2% dry weight), tea (3% dry weight), and chocolate (0.2% dry weight), contains 49.48% carbon, 5.15% hydrogen, 28.87% nitrogen, and 16.49% oxygen by mass and has a molar mass of 194.2g/mol. Determine the molecular formula of caffeine. C 8 H 10 N 4 O 2

47 Chemistry Chapter 3 Section 1 Homework Set pg 115: #42, 46, 53, 67, 72, 75, 81, 83, 86, 91 (10)

48 Chemistry- Unit 3 Section I Homework Set #1 pg 119: #24, 25, 41, 42, 43, 45, 47, 48, 49, 60, 64, 67, 68, 75, 76, 77, 84, 117, 135 (19) Lost bet: #119, 122