Chapter 7 - Problems
1) If the density of a gas at S.T.P. is 2.0 X 10-3 g/mL, what is the molar mass of the gas?
(45 g/mol)
Road Map:
D (g/mL) -------- D (g/L)  n(STP)  MM (g/mol)
MM = 2.0 X 10-3 g
X 1000 mL X 22.4 L = 45 g
mL
1 L mol
mol
2) If the molar mass of a gas is 71.0 g/mol, what mass of the gas will be contained in
a 12.25 L container at S.T.P.?
(38.8 g)
Road Map:
 MV
X MM
V(L) ----- n ------ xg
xg = 12.25 L X
mol
X
71.0 g = 38.8 g
1
22.4 L
mol
3) What volume will a sample of HCl occupy at S.T.P.?
(44.8 L)
Road Map:
xg  n
----------- V (STP)
V = 73.0 g X
mol
X
22.4 L = 44.8 L
36.5 g
mol
4) If 3.0 L (same temperature and pressure) of H2 react with O2 to form H2O,
what volume of O2 will be used?
(1.5 L)
2H2 +
2 mol
2V
3.0 L
Road map:
O2

1 mol
1V
V=?L
2H2O
2 mol
2V

Have
MV
X MV Want
V(H2) ---- n(H2) n (O2)-----V(O2)
V(O2) = 3.0 L X mol H2 X 1mol O2 X 22.4 L = 1.5 L
22.4 L 2 mol H2 1 mol O2
volume bridge
Road map:
V(H2) ---- V(O2)
V(O2) = 3.0 L H2 X 1V O2 = 1.5 L
2V H2
5) If 11.2 L of chlorine at S.T.P. were used, how many moles of NaCl would be produced?
(1.00 mol NaCl)
Cl2
+
1 mol
11.2 L
2Na 
2NaCl
2 mol
2 mol
? mol
 MV
Road map:
V(Cl2) ---n(Cl2) ---n(NaCl)
n(NaCl) = 11.2 L X mol Cl2 X 2 mol NaCl = 1.00 mol NaCl
1
22.4 L
1 mol Cl2
6) What volume of chlorine at S.T.P. would be used if 46.0 g of Na reacted?
ratio
data
MM
Road Map:
Cl2
+
2Na
1 mol
2 mol
V =? L 46.0 g
23.0 g/mol

2NaCl
2 mol

Have
MM
Mole Bridge X MV Want
xg(Na) --- n(Na) --- n(Cl2) ---- V(Cl2)
V(Cl2) = 46.0 g X mol Na X 1 mol Cl2 X 22.4 L = 22.4 L Cl2
23.0 g 2 mol Na mol Cl2
7) Calculate the volume of 617.5 g of methane gas, CH4, at S.T.P. (864 L)
V(CH4) = 617.5 g X mol X 22.4 L = 864 L
16.0 g
mol
8) How many grams of carbon dioxide will be required to fill 10 balloons at S.T.P., assuming that
each balloon has a volume of 450. mL? (8.84 g)
V = 450. mL = 0.450 L
V(10 balloons) = 0.450 L X 10 = 4.50 L
MM(CO2) = 44.0 g/mol

Road Map: Have
MV
X MM
V(mL)  V(L) ----------- V(CO2) ---- n(CO2) --- xg(CO2)
xg = 450. mL X 1 L
X 1 mol X
44.0 g = 8.84 g
1
1000 mL
22.4 L
mol
9) How many liters of hydrogen gas, measured at S.T.P. are required to react with
18.7 g of carbon, to produce methane, CH4? Include a balanced equation with your calculations.
(69.9 L)
Balanced Eqn
2H2 +
C

CH4
Mole Ratio
2 mol
1 mol
1 mol
Data
V= ? L
18.7 g
Road Map: xg(C) -- n(C) ---- n(H2)------ V(H2)
V(H2) = 18.7 g X mol C X 2 mol H2
X 22.4 L = 69.8 L
12.0 g
1 mol C
mol H2
10) How many moles of the following substances are contained in:
a) 10.6 L of SO2 (g) at S.T.P.
(0.473 mol)
10.6 L X mol = 0.473 mol
22.4 L
b) 0.120 L of NO2 (g) at S.T.P.
0.120 L X mol = 0.00535 mol
22.4 L
11) What is the mass, in grams, of:
a) 1.25 L of NH3(g) at S.T.P.
1.25 L X 1 mol X 17 g = 0.949 g
22.4 L mol
b) 3.45 mL of O2 (g) at S.T.P.
0.00345 L X 1 mol X 32 g = 0.00493 g
22.4 L mol
8
c) 1.00 X 10 L of H2 (g) at S.T.P.
1.00 X 108 L X 1 mol X 2.0 g = 8.93 X 106 g
22.4 L mol
***T***12- How many atoms are contained in:
a) 55.0 mL of N2O(g) at S.T.P.
Road Map:
MV
(5.36 X 10-3 mol)
(0.949 g)
(4.93 X 10-3 g)
(8.93 X 106 g)
21
(4.43 X 10 )
X A.N.
X #atoms
m/c
V(mL) V(L) ---- n ---- m/c -------------- atoms
21
0.055 L X 1 mol X 6.02 X 1023 m/c X 3 atoms = 4.43 X 10 atoms
22.4 L
mol
m/c
b) 83.0 mL of BF3 (g) at S.T.P.
Road Map:
Have
MV
21
(8.92 X 10 )
X A.N.
X #atoms Want
m/c
V(mL) V(L) --- n ----- m/c --------------- atoms
21
0.0830 L X 1 mol X 6.02 X 1023 m/c X 4 atoms = 8.92 X 10 atoms
22.4 L
mol
m/c
23
c) 15.0 L of Ar(g) at S.T.P.
(4.03 X 10 )
23
23
15.0 L X 1 mol X 6.02 X 10 atom = 4.03 X 10 atoms
22.4 L
mol