Page 201 - Theory and Problems of BEGINNING CHEMISTRY

190 GASES [CHAP. 12

12.34. What is the total pressure of a gas mixture containing H 2 at 0.173 atm, N 2 at 0.201 atm, and NO at
0.550 atm?
Ans. P total = P H 2 + P N 2 + P NO = 0.924 atm

12.35. What is the difference between Problems 12.32 and 12.34?
Ans. In Problem 12.34, molecules are involved. In Problem 12.32, uncombined atoms are involved. In both cases,
the same laws apply, and it is best to regard an uncombined atom of He, for example, as a monatomic
molecule.

12.36. The total pressure of a 125-mL sample of oxygen collected over water at 25 C is 1.030 atm. (a)How
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many moles of gas are present? (b) How many moles of water vapor are present? (c) How many moles
of oxygen are present?
PV (1.030 atm)(0.125 L)
Ans. (a) n = = = 5.26 × 10 −3 mol total
RT [0.0821 L·atm/(mol·K)](298 K)
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(b) The gas is composed of O 2 and water vapor. The pressure of the water vapor at 25 C, given in the last
part of Sec. 12.8 or in tables in your text, is 24 torr. In atmospheres:
1 atm

24 torr = 0.032 atm
760 torr
PV (0.032 atm)(0.125 L) −4
n H 2 O = = = 1.6 × 10 mol H 2 O
RT [0.0821 L·atm/(mol·K)](298 K)
(c) The pressure of the O 2 is therefore
1.030 atm − 0.032 atm = 0.998 atm
PV (0.998 atm)(0.125 L) −3
= = = 5.10 × 10
n O 2 mol O 2
RT [0.0821 L·atm/(mol·K)](298 K)
Check : 5.10 × 10 −3 mol + 0.16 × 10 −3 mol = 5.26 × 10 −3 mol

12.37. In Dalton’s law problems, what is the difference in behavior of water vapor mixed with air compared to
helium mixed with air?

Ans. The pressure of water vapor, if it is in contact with liquid water, is governed by the temperature only. More
water can evaporate or some water vapor can condense if the pressure is not equal to the tabulated vapor
pressure at the given temperature. Helium is a gas under most conditions and is not capable of adjusting its
pressure in the same way.

Supplementary Problems

12.38. Explain why gas law problems are not given with data to four or five significant figures.
Ans. The laws are only approximate, and having better data would not necessarily yield more accurate answers.

12.39. A mixture of gases contains He, Ne, and Ar. The pressure of He is 0.300 atm. The volume of Ne is 4.00 L. The
temperature of Ar is 27 C. What value can be calculated from these data? Explain.
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Ans. The temperature of Ar is the temperature of all the gases, since they are all mixed together. The volume of
Ne is the volume of each of the gases and the total volume, too. Therefore, the number of moles of He can
be calculated because its pressure is also known:
PV (0.300 atm)(4.00 L)
n = = = 0.0487 mol He
RT [0.0821 L·atm/(mol·K)](300 K)