SCH3U1
UNIT 3: MID-UNIT REVIEW QUESTIONS
Key Terms or Concepts:
- the mole
- % composition
- LCM and LDP
- empirical formula
- molecular formula
- combustion and hydrate analysis (to find empirical formula)
Formulas:
% _ composition 
Where:
mass of element
100
mass of compound
n
N
NA
M
m
n
N = number of particles (atoms or molecules)
NA = Avogadro’s Constant ( 6.022 x 1023 particles/mol )
m = mass (g)
M = molar mass (g/mol)
n = number of moles
MOLES
1. a) A balloon contains 0.5 mol of helium (He). How many atoms are there in the
balloon?
b) Another balloon contains 0.5 mol of oxygen gas. How many molecules of oxygen
(O2) are in this balloon?
c) How many atoms of oxygen are in the balloon mentioned in (b)?
2. Complete the chart:
compound
mass (g)
NaOH
Pb(NO3)4
Ca(OH)2
molar mass
(g/mol)
moles present
particles
present
60.0
1.505 x 1023
455.23
0.245
3. A chemist preparing to carry out a chemical reaction requires exactly 0.250 mol of
aluminum fluoride.
a) What is the molar mass (M) of aluminum fluoride?
b) How many grams of this compound should be measured?
4. The NASA space shuttle external tank contains a tank containing 629,340 kg of
oxygen gas and a second tank containing 106,261 kg of hydrogen gas. These
combine in an very exothermic reaction to generate water as a product. How many
moles of each gas are in these tanks?
5. What sample contains more gold atoms; a 100 g sample of gold (III) chloride (AuCl3)
or a 75 g sample of pure gold (Au)? Show your work.
LCM, LDP and PERCENT COMPOSITION
1. Calculate the percent by mass of each element present in ammonium phosphate
[(NH4)3PO4].
2. Early metal smelting of tin involved reacting tin oxides with carbon to produce the
metal and carbon dioxide:
tin (?) oxide (s) + carbon (s)
tin (s) + carbon dioxide (g)
a) If 452 g of tin oxide reacts with 36.0 g of carbon to produce 132 g of carbon dioxide,
what is the % composition of the tin oxide?
b) How much tin could be produced from the smelting of 1200 kg of the tin oxide?
EMPIRICAL AND MOLECULAR FORMULA
1. a) Find the empirical formula of a large organic compound that contains 43.89%
carbon, 7.38% hydrogen, and 48.73% oxygen by mass.
b) If the actual molecular mass of the compound is 656.6 g/mol, what is the molecular
formula?
COMBUSTION AND HYDRATE ANALYSIS
1. a) A compound containing C,H and O and weighing 5.714 g produced 14.414 g of
carbon dioxide and 2.529 g of water upon combustion. Find the empirical formula.
2. a) A 2.524 g sample of a compound contain C,H and O is subjected to combustion
analysis. 3.703 g of CO2 and 1.514 g of H2O are recovered. Determine the empirical
formula.
b) If one molecule of the compound contains 12 atoms of hydrogen, what is the
molecular formula?
3. A hydrate of zinc nitrate has the formula Zn(NO3)2 • xH2O. If the mass of 1 mol of
anhydrous zinc nitrate is 63.67% the mass of the hydrate form, what is the value of x?
Answers:
MOLES
1. a) 3.011 x 1023 atoms
b) 3.011 x 1023 molecules
c) 6.022 x 1023 atoms
2.
compound
mass (g)
molar mass
moles present
molecules
(g/mol)
(mol)
present
40.00
9.033 x 1023
NaOH
60.0
1.50
113.8
0.2499
Pb(NO3)4
455.23
1.505 x 1023
18.1
74.10
1.48 x 1023
Ca(OH)2
0.245
3 a) AlF3; 83.98 g/mol b) 21.0 g (to 3 sig. digits)
4. 1,97 x 107 mol O2; 5.26 x 107 mol H2
5. AuCl3 n = 0.330 mol; Au (s) n = 0.381 mol;  Au contains more gold.
LCM, LDP AND % COMPOSITION
1. %N = 28.19%; %H= 8,11%; %P = 20.77%; %O = 42.93%
2. a) 78.8% Sn; 21.2% O
b) 946 kg
EMPIRICAL AND MOLECULAR FORMULA
1. a) C6H12O5 b) C24H48O20
COMBUSTION AND HYDRATE ANALYSIS
1. C7H6O2
2. a) CH2O
b) C6H12O6
3. Zn(NO3)2 • 6H2O