Gas Stoichiometry
 2013, Sharmaine S. Cady
East Stroudsburg University
The gaseous phase is a physical state in which molecules have random motion and negligible
intermolecular interactions. Gases are easily compressed and flow from areas of high pressure to areas
of low pressures. Four physical quantities are used to describe a gas: pressure, volume, temperature,
and moles (or mass). The ideal gas law gives the relationship among these four variables:
R represents the universal gas constant, which may be expressed as 0.08206 Latm/molK or 62.36
Ltorr/molK.
In gaseous mixtures, each gas exerts a pressure independent of the other gases present. This
pressure is known as the partial pressure of the gas. The total of the individual partial pressures of each
gas in the mixture is the total pressure for the gaseous mixture. This is expressed by Dalton’s law, where
pn represents the partial pressure of a gas in the mixture:
A common practice in the laboratory is to collect a gas over water. The total pressure inside the
closed collection tube is adjusted to reflect the atmospheric pressure in the laboratory. The partial
pressure of the collected gas is determined from Dalton’s law by subtracting the water vapor pressure at
the temperature of the laboratory from the total pressure (atmospheric pressure). Water vapor
pressures at various temperatures are given on the next page.
Table 1. Vapor Pressure of Water
T, °C
19.1
19.2
19.3
19.4
19.5
19.6
19.7
19.8
19.9
20.0
20.1
20.2
20.3
20.4
20.5
20.6
20.7
20.8
20.9
21.0
21.1
21.2
21.3
21.4
21.5
21.6
21.7
21.8
21.9
22.0
p, torr
16.581
16.685
16.789
16.894
16.999
17.105
17.212
17.319
17.427
17.535
17.644
17.753
17.863
17.974
18.085
18.197
18.309
18.422
18.536
18.650
18.765
18.880
18.996
19.113
19.231
19.349
19.468
19.587
19.707
19.827
T, °C
22.1
22.2
22.3
22.4
22.5
22.6
22.7
22.8
22.9
23.0
23.1
23.2
23.3
23.4
23.5
23.6
23.7
23.8
23.9
24.0
24.1
24.2
24.3
24.4
24.5
24.6
24.7
24.8
24.9
25.0
p, torr
19.948
20.070
20.193
20.316
20.440
20.565
20.690
20.815
20.941
21.068
21.196
21.324
21.453
21.583
21.714
21.845
21.977
22.110
22.243
22.377
22.512
22.648
22.785
22.922
23.060
23.198
23.337
23.476
23.616
23.756
T, °C
25.1
25.2
25.3
25.4
25.5
25.6
25.7
25.8
25.9
26.0
26.1
26.2
26.3
26.4
26.5
26.6
26.7
26.8
26.9
27.0
27.1
27.2
27.3
27.4
27.5
27.6
27.7
27.8
27.9
28.0
p, torr
23.897
24.039
24.182
24.236
24.471
24.617
24.764
24.912
25.060
25.209
25.359
25.509
25.660
25.812
25.964
26.117
26.271
26.426
26.582
26.739
27.897
27.055
27.214
27.374
27.535
27.696
27.858
28.021
28.185
28.349
T, °C
28.1
28.2
28.3
28.4
28.5
28.6
28.7
28.8
28.9
29.0
29.1
29.2
29.3
29.4
29.5
29.6
29.7
29.8
29.9
30.0
30.1
30.2
30.3
30.4
30.5
30.6
30.7
30.8
30.9
31.0
Examples of calculations involving the collection of a gas over water follows.
p, torr
28.514
28.680
28.847
29.015
29.184
29.354
29.525
29.697
29.870
30.043
30.217
30.217
30.568
30.745
30.923
31.102
31.281
31.461
31.642
31.824
32.007
32.191
32.376
32.561
32.747
32.934
33.122
33.312
33.503
33.695
How to calculate the concentration of a bleach solution
A student measured 75.43 mL oxygen gas evolved over water when 4.00 mL of bleach reacted with
4.00 mL of H2O2 at an atmospheric pressure of 756.2 torr and a temperature of 22.0 °C. What is the
m/v% of NaOCl in the bleach solution?
The chemical reaction equation is
NaOCl(aq) + H2O2(aq)  O2(g) + NaCl(aq) + H2O(l)
Solution:
Write down the given values
with their units and what they
represent.
In this problem, three volumes, a pressure, and temperature are given. The
stoichiometric ratio is 1 mol NaOCl : 1 mol O2. The molar masses of Na, O, and Cl can
be obtained from the Periodic Table.
1 mol Na = 22.99 g/mol
1 mol O = 16.00 g/mol
1 mol Cl = 35.45 g/mol
molar mass NaOCl = 74.44 g/mol
4.00 mL = V bleach
4.00 mL = V H2O2
75.43 mL = V O2
P = 756.2 torr for air
p = 19.827 torr for water
t = 22.0 °C
T = 273.15 + 22.0 = 295.15 K
The water vapor pressure at 22.0 °C is obtained from the table above.
Write down the unknown and
its unit.
The m/v% of NaOCl in bleach is unknown.
m/v% NaOCl = ?
Develop a strategy to solve the
problem.
The definition of m/v% is
The g of NaOCl in bleach is obtained from the mol of O2 gas produced. The mol O2
gas produced is obtained from the ideal gas law and the partial pressure, volume,
and temperature variables. The partial pressure of O2 is obtained from Dalton’s law
and the atmospheric and water vapor pressures:
Values are not rounded off until the final answer is calculated.
Using the stoichiometry between NaOCl and O2, set up a dimensional analysis
problem to give g NaOCl/100 mL bleach.
Round off the answer to the
correct number of significant
digits and give the appropriate
units.
The volume of bleach has 3 significant digits while the other variables have 4; the
answer is rounded off to 3 significant digits. The answer is
m/v% = 5.62% NaOCl
Note that the volume of H2O2 is not used. NaOCl is the limiting reagent and
determines the amount of O2 produced. The H2O2 is in excess.
How to calculate the mass of a metal from gas volume
A student measured 0.951 L hydrogen gas evolved over water when with an excess of hydrochloric acid
reacted with the zinc in a penny at an atmospheric pressure of 748.3 torr and a temperature of 25.4 °C.
What is the mass of zinc in the penny?
The chemical reaction equation is
Zn(s) + 2 HCl (aq)  H2(g) + ZnCl2(aq)
Solution:
Write down the given values
with their units and what they
represent.
In this problem, the volume, a pressure, and temperature of a gas are given. The
stoichiometric ratio is 1 mol Zn : 1 mol H2. The molar mass of Zn can be obtained
from the Periodic Table.
1 mol Zn = 65.39 g/mol
0.951 L = V H2
P = 748.3 torr for air
p = 23.756 torr for water
t = 25.4 °C
T = 273.15 + 25.4 = 298.55 K
The water vapor pressure at 25.0 °C is obtained from the table above.
Write down the unknown and
its unit.
The mass of Zn in the penny is unknown.
mass Zn = ? g
Develop a strategy to solve the
problem.
The g of Zn in the penny is obtained from the mol of H2 gas produced. The mol H2
gas produced is obtained from the ideal gas law and the partial pressure, volume,
and temperature variables. The partial pressure of H2 is obtained from Dalton’s law
and the atmospheric and water vapor pressures:
Values are not rounded off until the final answer is calculated.
Using the stoichiometry between Zn and H2, set up a dimensional analysis problem
to give g Zn in the penny.
Round off the answer to the
correct number of significant
digits and give the appropriate
units.
The volume of H2 has 3 significant digits while the other variables have 4; the answer
is rounded off to 3 significant digits. The answer is
mass Zn = 2.42 g