The Foundations of Chemistry

Solution

(a) For O 2 ,

P 1 V 1 P 2 V 2 or P2,O 2 

P

V

1 V

2

^1 15.0 atm

For N 2 ,

P 1 V 1 P 2 V 2 or P2,N 2 

P

V

1 V

2

1

12.0 atm

The total pressure is the sum of the partial pressures.

PtotalP 2 ,O 2 P 2 ,N 2 15.0 atm12.0 atm 27.0 atm

(b) XO 2 0.556

XN 2 0.444

As a check, the sum of the mole fractions is 1.

You should now work Exercise 62.

Some gases can be collected over water. Figure 12-7 illustrates the collection of a sample

of hydrogen by displacement of water. A gas produced in a reaction displaces the denser

water from the inverted water-filled container. The pressure on the gas inside the collec-

tion container can be made equal to atmospheric pressure by raising or lowering the

container until the water level inside is the same as that outside.

One complication arises, however. A gas in contact with water soon becomes saturated

with water vapor. The pressure inside the container is the sum of the partial pressure of

the gas itself plusthe partial pressure exerted by the water vapor in the gas mixture (the

vapor pressureof water). Every liquid shows a characteristic vapor pressure that varies

only with temperature, and notwith the volume of vapor present, so long as both liquid

12.0 atm



27.0 atm

P 2 ,N 2



Ptotal

15.0 atm



27.0 atm

P2,O 2



Ptotal

32.0 atm3.00 L



8.00 L

24.0 atm5.00 L



8.00 L

Notice that this problem has been
solved without calculating the number
of moles of either gas.

Gases that are soluble in water or that
react with water cannot be collected by
this method. Other liquids can be
used.

The partial pressure exerted by the
vapor above a liquid is called the vapor
pressure of that liquid. A more
extensive table of the vapor pressure of
water appears in Appendix E.

460 CHAPTER 12: Gases and the Kinetic–Molecular Theory

Figure 12-7 Apparatus for
preparing hydrogen from zinc and
sulfuric acid.

Zn(s)2H(aq)88n
Zn^2 (aq)H 2 (g)

The hydrogen is collected by
displacement of water.