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Solving Problems: A Chemistry Handbook
Chemistry: Matter and Change
137
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 13
What volume of NH 3 gas, measured at 78°C and a pressure of 1.66
atm, will be needed to produce 5.00 ϫ 10 3 g of (NH 4 ) 2 SO 4 ?
First, you need to compute the number of moles represented by
5.00 ϫ 10 3 g of (NH 4 ) 2 SO 4 . Using atomic mass values from the
periodic table, you can compute the molar mass of (NH 4 ) 2 SO 4 to be
132.14 g/mol.
n (NH 4 ) 2 SO 4
ϭ
ϭ 37.84 mol (NH 4 ) 2 SO 4
Next, determine the number of moles of NH 3 that must react to
produce 37.84 mol (NH 4 ) 2 SO 4 .
n NH 3
ϭ 37.84 mol (NH 4 ) 2 SO 4 ϫ
ϭ 75.68 mol NH 3
Finally, use the ideal gas law equation to calculate the volume of
75.68 mol NH 3 under the stated conditions. Solve the equation for
V, the volume to be calculated.
PV ϭ nRT; V ϭ
Convert the temperature to kelvins, substitute known quantities into
the equation, and compute the volume.
V ϭ
ϭ
ϭ1310 L
Notice that the values for the molar mass of (NH 4 ) 2 SO 4 and the
number of moles of NH 3 have more than three significant figures,
whereas the calculated volume has only three. When you do a
problem in a stepwise way, you should maintain at least one extra
significant figure in the intermediate values you calculate. Then,
round off values only at the end of the problem.
Practice Problems
24. A piece of aluminum with a mass of 4.25 g is used to generate
hydrogen gas by the following method.
2Al(s) ϩ 6HCl(aq) 0 2AlCl 3 (aq) ϩ 3H 2 (g)
The hydrogen is collected at a temperature of 15°C and a
pressure of 94.4 kPa. What volume of hydrogen is produced?
75.68 mol NH 3 ϫ 0.0821ᎏ
m
L и
o
a
l и
tm
K
ᎏ ϫ 351 K
1.66 atm
nRT
ᎏ
P
nRT
ᎏ
P
2 mol NH 3
ᎏᎏ
1 mol (NH 4 ) 2 SO 4
5.00 ϫ 10 3 g (NH 4 ) 2 SO 4
ᎏᎏᎏ
132.14 g/mol (NH 4 ) 2 SO 4