Download Unit 2 – Quantities Review

Unit 2 – Chapter 4 – Quantities Review
Calculating Molar Mass
 Molar mass = mass of 1 mol (6.02x1023 molecules/particles) of a
compound or element
 How do you calculate the molar mass?
o Write the chemical formula for the compound (if not given)
o Add the average atomic mass for each element within the
compound
 Remember to multiply by the average atomic mass for each
element by any subscripts that are present
 ie. H2  2(1.01) = 2.02 g/mol
o Unit for molar mass is g/mol
Practice Problems:
1. What is the molar mass of ammonium phosphate, (NH4)3PO4?
2. What is the molar mass of calcium hydroxide, Ca(OH)2(s)?
3. What is the molar mass of chlorine, Cl2(g)
4. What is the molar mass of the polyatomic ion, hydroxide?
5. If an unknown element has a molar mass of 196.97 g/mol. What element
could it be?
6. What does the term diatomic mean?
a.
b.
Name seven elements that form diatomic molecules.
How many molecules are present in one mole of any diatomic
element?
Converting from mass (g) to moles
 Take the number of grams and divide by the molar mass
 Eg. Calculate the number of moles in 10 g of H2(g)
mass of H2 = 10 g
molar mass H2 = 2.02 g/mol
nH2 = ?
10 g H2 x 1 mol H2 = 4.95 mol H2
2.02 g H2
Therefore, there are 4.95 mol of H2 in 10g of H2(g)
Practice Problems:
7. Convert a mass of 2.5 g of table salt (sodium chloride) to an amount in moles.
8. Convert a mass of 1.0 kg of glucose, C6H12O6(s), to an amount in moles.
9. What is the amount in moles of 25.0 g of oxygen gas?
Converting from moles to mass (g)
 Take the number of moles and multiply by the molar mass
 Eg. Calculate the mass of 0.400 moles of Carbon(s)
nC = 0.400 moles
molar mass C = 12.01 g/mol
mass of C(s) = ?
0.400 mol C x 12.01 g C = 4.80 g C
1 mol C
Practice Problems:
10. Magnesium hydroxide is a base that is used in some antacids. What is the
mass in grams of 0.45 mol of magnesium hydroxide?
11. Ammonia is a gas that, dissolved in water, is used as a cleaning agent.
Convert 87 mmol of ammonia, NH3(g), into mass in grams. (Hint: 1 mol =
1000 mmol, you will need to convert from mmol to mol)
12. 1,4-benzenedioic acid, C8H6O4(s),is a raw material used in the manufacture of
Dacron, a synthetic fibre. What is the mass in grams of 63.28 mol of 1,4benzenedioic acid?
13. If a patient is prescribed 1.0 × 10–3 mol of acetylsalicylic acid (Aspirin,
C9H8O4(s)), what mass of Aspirin should she take?
Calculating the number of molecules in a sample
a. Use Avagadro’s Number 6.02 x 1023 molecules/mol
b. Can only be used to convert from moles to molecules
(1)
You must convert grams to moles first!
Calculating the number of atoms in a sample
 Find number of molecules first, using Avagadro’s Number
 Multiply number of molecules by total number of atoms in 1
molecule
Practice Problems:
14. How many molecules are there in 3.00 mol of carbon dioxide?
15. How many atoms are there in 0.500 mol of Ar(g)?
16. How many atoms are there in 15 mol of solid carbon dioxide, in dry ice?
17. How many molecules are there in 15 g of ammonia gas, in household
cleaners?
Percent Composition
 Find molar mass of the compound
 Divide the molar mass of each element by the molar mass of the
compound
Ex. Determine the percent composition of calcium sulfate.
Molar mass of CaSO4 = 40.08 + 32.07 + 4(16.00) = 136.11 g/mol
Percent of Ca = mCa____ x 100% = 40.08g/mol x 100% = 41.40%
mCaSO4
136.11 g/mol
Percent of S = mS____ x 100% = 32.07 g/mol x 100% = 23.56%
mCaSO4
136.11 g/mol
Percent of O = mO____ x 100% = 64.00 g/mol x 100% = 47.02%
mCaSO4
136.11 g/mol
Practice Problems
18. Determine the percentage composition of sodium carbonate, Na2CO3.
19. Calculate the percentage composition by mass of sulfuric acid,H2SO4(aq), used
in car batteries.
20. Calculate the percentage by mass of magnesium in magnesium hydroxide,
Mg(OH)2(s), used in some antacids.
21. Iron and oxygen combine to form two different compounds. The formulas of
the compounds are FeO(s) and Fe2O3(s). Calculate the percentage composition
of each compound.
Determining Empirical Formulas
 Find percent composition, if not given in question.
 Assume 100g of the compound.
 Find mass of each element present. (This is the same as the %
composition)
o ie. If the compound is 25% O, Then in 100 g of compound there is
25g of O.
 Convert mass (g) to moles for each element.
 Divide the number of moles for each element by the smallest number
of moles
o ie. If the mole ratio (Na:Cl:O) is 0.940:0.939:2.82.
o Divide by 0.939 to obtain the lowest ratio 0.940 : 0.939 : 2.82
0.939 0.939 0.939
 If values are within 0.05 of a whole number, round to a whole number
o From above, we get 1.00: 1.00 : 3.00, since all are within 0.05 of a
whole number
o We get the mole ratio of Na:Cl:O = 1:1:3.
 If values are not close to a real number multiply all values by the same
coefficient to end up with whole numbers.
Ex. If the ratio was 1:2:1.33, multiply all numbers by 3 to get 1:6:4
Practice Problems
22. The percentage compositions of two antibiotics are given below. Find the
empirical formula of each:
a. chloromycetin: 40.87% carbon, 3.72% hydrogen, 8.67% nitrogen, 24.77%
oxygen, and 21.98% chlorine.
b. sulfanilamide: 41.86% carbon, 4.65% hydrogen, 16.28% nitrogen,18.60%
oxygen, and 18.60% sulfur.
23. Phosphorus combines with oxygen to form two oxides. Find the empirical
formula for each oxide of phosphorus if the percentage composition for each
is:
a. 43.6% oxygen
b. 56.6% oxygen
24. Propane is a hydrocarbon that is used as fuel in barbecues and some cars. In
a 26.80-g sample of propane, 4.90 g is hydrogen and the remainder is
carbon. What is the empirical formula of propane?
(Hint – use the given masses to determine % composition)
Determining Molecular Formulas
 You must know the molecular mass of the final compound to determine
molecular formula (you will be given this in the question)
 Determine the empirical formula
 Determine the molar mass of the empirical formula
 Divide the molar mass of the final compound by the molar mass of the
empirical formula
 Multiply the subscripts of the empirical formula by the resulting number
Practice Problems
25. A key ingredient of nail polish remover is found to have the following
percentage composition by mass: 62.0% carbon, 10.4% hydrogen, and
27.5% oxygen. If its molar mass is 58.1 g/mol, what is its molecular formula?
26. Analysis of an air pollutant reveals that the compound is 30.4% nitrogen and
69.6% oxygen, by mass. The mass spectrograph for the pollutant shows that
its molar mass is 92.0 g/mol. Determine the
molecular formula and the chemical name of the polluting compound.
Answers:
1. 149.12 g/mol.
2. 74.10 g
3. 70.90 g
4. 17.01 g
5. Gold (Au)
6. 2 atoms of the same element covalently bonded to form a molecule
a. Cl, F, I, Br, H, O, N
b. Avagadro’s number, 6.02x1023 molecules
7. 0.043 mol
8. 5.6 mol
9. 0.781 mol
10. 26 g
11. 1.5 g
12. 10.51 kg
13. 0.18 g
14. 1.81 × 1024 molecules
15. 3.01 × 1023 atoms
16. 9.0 x 1024 molecules
17. 5.3 x 1023 molecules
18. 43.38% sodium, 11.33% carbon, and 45.29% oxygen.
19. 2.1% H, 32.7% S, 65.2% O
20. 41.6% Mg
21. FeO(s) = 77.7% Fe, 22.3% O; Fe2O3(s) = 69.9% Fe, 30.1% O
22. a. C11H12N2O5Cl2 b. C6H8N2O2S
23. a. P2O3 b. P2O5
24. C3H8
25. C3H6O
26. N2O4