CHEM 115 Masses and Moles

Transcription

1 CHEM 5 Masses and Moles Lecture 5 Prof. Sevian Facilitated Study Groups Tuesdays 3:20-4:20 Wednesdays :00-2:002:00 Both FSGs are in Science, st floor, room 89 (also known as the chemistry conference room) If you elect to join one, please commit to attending it for the entire semester 2 H. Sevian

2 Agenda Practice some chemical nomenclature What is a mole? (money analogy) Converting between ways of measuring quantity of matter (mass moles) Empirical formulas Determining % composition (by mass) of each element given a chemical formula Determining empirical formula given % composition (by mass) of elements in a compound Determining molecular formula if given empirical formula and molar mass Chemical equations Terminology: reactants, products Using conservation of matter to balance chemical equations 3 A last note about naming compounds: Families of polyatomic ions Chlorine family ClO (a.k.a. a OCl ) ClO 2 ClO 3 ClO 4 Chlorine family hypochlorite chlorite chlorate perchlorate Bromine family BO BrO (a.k.a. OBr ) ) BrO 2 BrO 3 BrO 4 Bromine family hypobromite bromite bromate perbromate H. Sevian 2

3 Clicker question # for today What is the name of (NH 4 ) 2 S 2 O 3? A. Nitrogen tetrahydrogen disulfur trioxide B. Diammonium thiosulfate C. Ammonium sulfoxide D. Ammonium thiosulfate 5 Clicker question #2 What is the name of FeCO 3? A. Iron carbonate B. Iron (I) carbonate C. Iron (II) carbonate D. Iron (III) carbonate E. Iron carboxide 6 H. Sevian 3

4 Clicker question #3 What is the formula for copper (II) bromite? A. Cu 2 BrO B. Cu 2 BrO 2 C. CuBrO 4 D. Cu(BrO 2 ) 2 E. Cu(BrO) 2 7 Clicker question #4 What is the name of H 2 C 2 O 4? A. Dihydrogen dicarbon tetroxide B. Hydrogen oxalate C. Oxalic acid D. Carboxylic acid E. Hydrogen carbon oxide 8 H. Sevian 4

5 What is a Mole? Convenient way of counting particles Analogous to dozen, pair, gross, case, ream One mole of particles always contains the same number of particles, regardless of the identity of the particle mole of = particles of units e.g., mol Ar = Ar atoms mol O 3 (g) = O 3 molecules mol NaCl (s) = NaCl units 9 Moles scale from atoms How many atoms of O are in one unit of each of the following compounds? H 2 SO 4 Ca(NO 3 ) 2 C 2 H 22 O How many moles of O atoms are in mol of each of the following compounds? H 2SO 4 Ca(NO 3 3) 2 C 2 H 22 O Possible conversions are: 6 mol O atoms or mol Ca(NO 3 ) 2 mol Ca(NO 3 ) 2 6 mol O atoms H. Sevian 5

6 What is the Mass of a Mole? Different kinds of particles have different masses Since the quantity of particles in a mole is the same for any particle, the mass of a mole of particles varies depending on the identity of the particles By definition, 2 mol of C particles = 2 exactly 6 g Other molar masses always in reference to definition. 2 Molar Mass Molar mass means the mass of one mole For monatomic elements, use atomic weight on the Periodic Table (remember what atomic weight means?) Examples: What is the mass of.000 mol of Ar? g Ar.000 mol Ar = g Ar mol Ar What is the mass of 2.5 mol of Xe? 3.3 g Xe 2.5 mol Xe = g Xe mol Xe 330 g Xe 3 H. Sevian 6

7 What is the amount of money here? How did you figure this out? What is the amount of amu s here? Answer: 60 amu How did you figure this out? 5 H. Sevian 7

8 Actually, the masses aren t integers H =.008 C = 2.0 C = 2.0 O = 6.00 Mass of one molecule of CH 3 COOH is amu H =.008 H =.008 H =.008 O = 6.00 Mass of mole of CH 3 COOH molecules is grams Important note: Since there is always the same number of particles in a mole, when you determine the amu s of a unit, you are determining the mass in grams of a mole of that unit. 6 Q: How do mass mole conversions work? A: Just like money How much does mole of this (imaginary) chemical cost? 76 or 76 mol mol 76 How much would it cost to buy 2 moles of the chemical? 2 moles 76 mol = 52 H. Sevian 8

9 Moles and masses, so far Mass (in grams) of a mole is the same number as the mass (in amu) of one unit in the mole Use the average atomic weights on the periodic table as masses of individual atoms There are many names for this quantity: Molar mass Molecular weight (when the unit is a molecule) Formula weight (when the unit is the simplest ratio of ions in an ionic crystal) 8 More on Molar Mass What is the mass of mol of CH 3 COOH? H =.008 H =.008 C = 2.0 C = 2.0 O = 6.00 Molar mass = g/mol H =.008 H =.008 O = g mol CH3COOH = g CH mol 0 3 COOH 9 H. Sevian 9

10 More on Molar Mass For ionic compounds, sometimes also called formula weight To determine molar mass (or formula weight) of one unit of ionic compound: sum the parts Formula weight of NH 4 Cl crystals: One unit is (one NH 4+ ) plus (one Cl ) N = 4.0 H =.008 H =.008 Cl = H =.008 H =.008 Molar mass = g/mol From Chemistry & Chemical Reactivity 5 th edition by Kotz / Treichel. C Reprinted with permission of Brooks/Cole, a division of Thomson Learning: Fax Hydrated Ionic Compounds How many moles are in 5.55 g of BaCl 2 2H 2 O? One unit of ionic formula consists of: One Ba 2+ ion = 37.3 Two Cl ions = Two H 2 O molecules = Formula weight = g/mol mol.55 g BaCl2 2H2O = mol BaCl2 2H g 5 2 O H. Sevian 0

11 Conversions possible so far (look for opportunities to use these in dimensional analysis) Moles mass (grams) Moles of atoms moles of a unit (e.g., molecule, neutral ionic crystal formula) Moles of something particles in the something 22 Combining molar mass and numbers of atoms Example How many atoms of hydrogen are in.63 grams of water (H 2 O)?? another? atoms? something thing of H.63 g H 2 O =? atoms of H? g H 2 O? something? another thing 3 sig figs ? molecules? 2 atoms? mol H 2 O of H.63 g H 2 O 2 O of H = atoms? atoms of of HH 8.02? g H 2 Og? mol H 2 O? molecule H 2 O of H 2 O 3 sig figs H. Sevian

12 How Many Molecules? Too many to count! How many molecules of water are in a 2.00-L bottle that is filled with water? Water has a density of.000 g/ml L Start End 200L 2.00? ? molecules 0 25 molecules of H 2 O 000 ml.000 g mol molecules of H = 2 O? L L ml 8.02 g mol Composition of a Can be: Composition of compounds in a mixture Composition i of elements in a compound Composition of ions or molecules in a complex ion Usually composition is given in mass percent (also called weight %) Use the chemistry definition of percent part % = 00% whole Know the difference between fraction and percent a fraction can be expressed as a percent by multiplying by 00 ( percent means part out of 00) Fraction is the same as 75.85% H. Sevian 2

13 Composition of a Mixture Baking powder is a mixture of ingredients. A sample of baking powder contains 3.50 g of calcium hydrogen phosphate (CaHPO 4 ),.50 g of sodium bicarbonate b (NaHCO 3 ), and.00 g of silicon dioxide (SiO 2 ). Calculate the percent composition by mass. 3.50g A picture of % CaHPO the mixture 4 = 00% = 58.3% g (to assist in estimating) NaHCO 3 %NaHCO 3.50 g = 00% = 25.0% 6.00g SiO 2 %SiO 2 = 00 ( )% = 6.7% CaHPO 4 What does % composition mean? 6 % Composition by Mass 2 2 fraction C = = 40.% fraction H = = 6.7% fraction O = = 53.3% 60 Note: These are approximate atomic masses, for the purpose of demonstrating % composition. When actually calculating % composition, use the values from the Periodic Table. 40% + 6.7% % = 00% check H. Sevian 3

14 What does % composition mean? Acetic Acid: Percent Composition by Mass C 40.0% O 53.3% H 6.7% Composition of a Compound What is the composition by mass of acetic acid (CH 3 COOH)? %C = 00% = 40.00% C = 2.0 H =.008 H =.008 C = 2.0 O = g %H = 00% = 6.74% 60.05g 32.00g %O = 00% = 53.29% 60.05g H =.008 H =.008 O = 6.00 Recall from earlier: Molar mass = g/mol Check : = 00.00% H. Sevian 4

15 Composition of a Hydrated Compound Heating barium chloride dihydrate (BaCl 2 2H 2 O) drives off the water, leaving the anhydrous compound (BaCl 2 2) ). The chemical reaction is BaCl 2 2H 2 O (s) BaCl 2 (s) + 2 H 2 O (g) If you begin with a 0.0 g sample of the hydrated compound, what mass of water will be lost? % H 2 O = 00% = 7.379% mass of H2O in sample = 7.379% of 0.0 g = g = 0.738g 8.02 Formula weight = g/mol Problem solving strategies Ascertain whether the problem looks like a percent composition or empirical formula problem - there are only two directions to go:. converting mass to moles (and then ratios of moles), or 2. converting (ratios of) moles to mass (and then % by mass) What information is given? What information is sought? Strategize (it is helpful to draw pictures): masses or percentages given: this is a mass to moles problem chemical formula given: this is a moles to mass problem 32 H. Sevian 5

16 Chemical Compounds and Mass Chemical formula to percent composition Need to determine parts and whole Use definition of percent Going the other direction Percent composition alone is not enough information to determine molecular formula C 2 H 4 C 4 H 8 C6 H 2 All three of these have 4.37% H and 85.63% C by mass 33 Empirical Formula Lowest whole number ratio of elements in a chemical formula Chemical formula Empirical formula C 2 H 4 CH 2 C 4 H 8 CH 2 C 6 H 2 CH 2 C 6 H 2 O 6 Na 2 C 2 O 4 CH 3 COOH H 2 O 2 H 2 O 34 H. Sevian 6

17 Chemical Compounds and Mass Chemical formula to percent composition Ratio of moles Percent by mass Percent composition (or relative masses) to empirical formula Percent by mass Ratio of moles 35 Step : % Composition to Empirical Formula Analysis of a particular compound shows that it is composed of X% carbon, Y% hydrogen, and Z% oxygen. What is the compound s empirical formula? M A S S (grams) M O L E S (mol) X grams are carbon 00 grams Y grams are hd hydrogen 00 grams Z grams are oxygen 00 grams specific whole number ratio of moles C : moles H : moles O is a : b : c empirical formula is C H O a b c H. Sevian 7

18 M A S S (grams) M O L E S (mol) Step : % Composition to Empirical Formula Analysis of a particular compound shows that it is composed of 73.4% carbon, 7.37% hydrogen, and the remainder is oxygen. What is the compound s empirical formula? 73.4 g C mol C = mol C 2.0 g 7.37 g H mol H = 7.3 mol H.008 g 9.49 g O mol O =.28 mol O 6.00 g specific whole number moles C : moles H : moles O is : 7.3 : = : = 5 : 6 : so, empirical.28 :.28 formula is C ratio of 5H6O Variations on Determining Empirical Formula Start with % composition of elements in a compound Start t with masses of elements in a compound Start with % or masses of parts of a hydrated compound (e.g., determine n in CuSO 4 nh 2 O, given mass of CuSO 4 and mass of H 2 O lost when compound is heated) 39 H. Sevian 8

19 Step 2: Empirical Formula to Molecular Formula A hydrocarbon has 85.63% carbon by mass. What is its empirical formula? CH 2 What else do you need to know to determine molecular formula? C 2 H 4 C 4 H 8 C6 H 2 (All three have 4.37% H and 85.63% C by mass) How many empirical formulas? Empirical Unit CH 2 Mass is (.008) = 4.03 g/mol C C2 H C 4 4 H 8 C6 H 2 2 (CH 2 ) = 2 (4.03) = g/mol 4 (CH 2 ) = 4 (4.03) = 56.2 g/mol 6 (CH 2 ) = 6 (4.03) = 84.8 g/mol H. Sevian 9

20 What we have learned so far Moles are a counting group Mass and moles are two different measures of the quantity of matter Mass is what you measure in the lab (use a scale or mass balance) Moles are a count of how many particles (can t be measured directly) The two are connected because each particle has a mass which depends on what the particle is made of There are lots of ways to torture chemistry students with using mass to moles conversions What is the molar mass of a compound? How many moles are in a particular mass of compound? (and vice versa) What is the % composition by mass of the different elements in a particular compound? What is the empirical formula of a compound that has a particular set of % composition (by mass) of elements? What is the molecular formula if you know empirical formula and molar mass? Combinations of the above Clicker question #5 What kind of problem is this - what strategy would you use? Serotonin is a compound that conducts nerve impulses in the brain. It contains 68.2 mass percent C, 6.86 mass percent H, 5.9 mass percent N, and the rest O. Its molar mass is 76 g/mol. Determine its molecular formula. A. Take the ratio of 68.2 : 6.86 : 5.9 : whatever part O, convert to whole numbers to get molecular formula B. Divide each percentage (as grams) by 76 to get moles, then take ratio C. Assume 00 g so % s are grams, convert to moles, then take ratio of moles D. Assume mol, calculate mass of each element in 79 g, then convert those masses to number of moles to get ratio of moles 43 H. Sevian 20