Physical Chemistry Third Edition

334 7 Chemical Equilibrium

Solution

From the equilibrium of Eq. (7.5-7b)

a(CO 2 (aq))

a(H 2 CO 3 )

2. 58 × 10 −^3

(389)a(H 2 CO 3 )

a(H 2 CO 3 )+a(CO 2 (aq))(389)a(H 2 CO 3 )

From the equilibrium of Eq. (7.5-7c)

a(H+)a(HCO3−)(1. 70 × 10 −^4 )a(H 2 CO 3 )

a(H+)a(HCO3−)

1. 70 × 10 −^4

389

(a(H 2 CO 3 )+a(CO 2 (aq)))

K(effective)

a(H+)a(HCO3−)

a(H 2 CO 3 )+a(CO 2 (aq))



1. 70 × 10 −^4

389

 4. 37 × 10 −^7

This equilibrium constant is near 10−^7 , so that this equilibrium can provide buffering

near pH7.

Exercise 7.14

a.Find the pH of a solution produced by equilibrating water with gaseous carbon dioxide at

760 torr at 25◦C. Assume all activity coefficients are equal to unity.

b.Repeat the calculation of part b, using the Davies equation to estimate activity coefficients.

The pH of human blood can be raised by removing dissolved carbon dioxide from

the blood through hyperventilation (rapid breathing). The reaction of Eq. (7.5-7b) is

catalyzed by an enzyme, carbonic anhydrase, so that equilibrium is established rapidly.

If the molality of dissolved carbon dioxide is lowered below its normal value, the

reaction of Eqs. (7.5-7b) and (7.5-7c) shift to the left, raising the pH of the blood

and producing a feeling of “light-headedness.” A person who has hyperventilated is

often told to breathe into and out of a paper bag. Since exhaled air is about 4% carbon

dioxide, this practice increases the level of carbon dioxide in the blood and alleviates the

symptoms. The body also attempts to correct the situation by increasing the excretion

of the bicarbonate ion in urine.

PROBLEMS

Section 7.5: Buffer Solutions

7.41 Find the pH of a buffer solution made from 0.300 mol of
acetic acid, 0.500 mol of sodium acetate, and 1.000 kg of

water. The acid ionization constant of acetic acid is equal

to 1. 75 × 10 −^5.

a.Use the Henderson–Hasselbalch equation.