1. Calculate the number of atoms of silicon in 0.367 moles of silicon.
6.10 x atoms Si
2.21 x atoms Si
2.21 atoms Si
1.64 x atoms Si
What do you need ?
Avogadro’s Number
1 mole = 6.02 x atoms
0.367 mol x x atoms / 1 mol =
2.21 x atoms Si

2. Calculate the molar mass of potassium phosphate.
134 g/mol
86.07 g/mol
g/mol
g/mol
What do you need ?
1) Formula of potassium phosphate:
K3PO4
2) Atomic Weights
(molar mass)
K = 39.10, P = 30.97, O = 16.00
3(39.10) (16.00) =
g/mol

3. Calculate the mass in grams of 4.00 moles of sulfur.
What do you need ?
Atomic Weight sulfur (S) = 32.07
(molar mass) = g/mol
4 mol sulfur x g/mol sulfur =
128.3 g

4. How many moles is 146.3 grams of nickel?
What do you need ?
Atomic Weight (Ni) = 58.69
(molar mass Ni) = g/1 mol
146.3 g Ni x 1 mol Ni / g Ni =
2.493 mol

5. Calculate the molar mass of methane gas if 1
Calculate the molar mass of methane gas if 1.00 g of methane occupies a volume of 1.39 L at STP.
22.4 g/mol
16.1 g/mol
1.39 g/mol
0.719 g/mol
What do you need ?
1 mole of a STP = 22.4 L
1 mol / 22.4 L = x mol / 1.39 L
x mol = 1.39 L . mol / 22.4 L
x mol = mol
1.00 g / mol =
16.1 g/mol

6. Calculate the percentage composition for all the elements in nicotine, C10H14N2.
What do you need ?
Molar mass C10H14N2
C = 12.01, H = 1.01, N = 14.01
10(12.01) + 14(1.01) + 2(14.01) = g/mol
120.1g/mol g/mol g/mol
120.1/ / /
x 100 = x 100 = x 100 =
74.02% % %

7. Freon was used commonly as a refrigerant until CFCs were found to deplete the ozone layer. An analysis of freon gave the following data: 9.93% carbon, 58.64% chlorine, and 31.43% fluorine. What is the empirical formula of freon, CxClyFz?
What do you need ?
Atomic masses:
C = 12.01, Cl = 35.45, F = 19.00
Calculate respective moles:
9.93g/12.01 g/mol * g/ g/mol * g /19.00 g/mol
0.827mol mol mol
CxClyFz = C0.827Cl1.654 F1.654
C1Cl2F2
CCl2F2

8. In an experiment, 5. 00 g of tungsten reacts with air to give 6
In an experiment, 5.00 g of tungsten reacts with air to give 6.31 g of tungsten oxide.
What is the empirical formula of tungsten oxide?
W3O W5O WO WO
What do you need ?
Equation for Reaction:
x W(s) + y/2 O 2 (g) WxOy (s)
Mass of O2
6.31 g g = 1.31g
Atomic masses: W = , O = 16.00
Calculate respective moles:
5.00/ g/mol * g /16.00 g/mol
0.0272mol mol
WxOy = W O
W1 O 3.01
W O 3

9. A student calculated the relative number of moles for several different compounds as follows. What are the correct empirical formulas?
Al1 O 1.511
Al2 O 3 aluminum oxide
K3.012P1O3.973
K3PO4 potassium phosphate
N1 O 2.501
N2 O5 dinitrogen pentoxide
C1.002H1.331O1.006
C3H4O3 pyruvic acid
C1.012H2.001O1.009
CH2O formaldehyde

10. A student calculated an empirical formula of CH2O for an unknown white solid. The solid could possibly be several different compounds: formaldehyde, lactic acid, erythrose, ribose or glucose. Mass spectrometry results indicated a molar mass of g/mol. What is the correct molecular formula of the solid?
CH2O formaldehyde
C3H6O3 lactic acid
C4H8O4 erythrose
C5H10O5 ribose
C6H12O6 glucose
What do you need ?
Molar mass CH2O = g/mol
C = 12.01, H = 1.01, O = 16.00
Compare molar masses: g/mol / g/mol = 3
3 x (C1H2O1) = C3H6O3 lactic acid