Page 266 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 17] ACID-BASE THEORY 255

17.11. What concentration of acetic acid, HC 2 H 3 O 2 , is needed to give a hydronium ion concentration of 4.0 ×
−5
10 −3 M? K a = 1.8 × 10 .
Ans. HC 2 H 3 O 2 + H 2 O ←− H 3 O + + C 2 H 3 O 2 −
−→
Initial x 0 0
Change −0.0040 0.0040 0.0040
Equilibrium x − 0.0040 = x 0.0040 0.0040
∼
+
−
[H 3 O ][C 2 H 3 O 2 ] (0.0040) 2
K a = = = 1.8 × 10 −5
[HC 2 H 3 O 2 ] x
x = 0.89 M

AUTOIONIZATION OF WATER
17.12. According to Le Chˆatelier’s principle, what does the H 3 O concentration from the ionization of an acid
+
(strong or weak) do to the ionization of water?
Ans. 2H 2 O ←− H 3 O + OH −
−→
+
+
The presence of H 3 O from an acid will repress this water ionization. The water will ionize less than it does
+
in neutral solution, and the H 3 O generated by the water will be negligible in all but the most dilute acid.
−
The OH generated by the water will be equally small, but since it is the only OH present, it still has to be
−
considered.
17.13. Calculate the hydronium ion concentration in 0.50 M NaCl.
Ans. Since both Na and Cl are feeble, neither reacts with water at all. Thus, the hydronium ion concentration
−
+
−7
in this solution is the same as that in pure water, 1.0 × 10 M.
17.14. What does a 0.20 M HNO 3 solution contain?
−
+
−
Ans. It contains 0.20 M H 3 O , 0.20 M NO 3 ,H 2 O, and a slight concentration of OH from the ionization of the
water.
17.15. What does a 0.20 M HC 2 H 3 O 2 solution contain?
Ans. It contains nearly 0.20 M HC 2 H 3 O 2 , a little H 3 O and C 2 H 3 O 2 ,H 2 O, and a slight concentration of OH −
−
+
from the ionization of the water.

THE pH SCALE
17.16. Write an equilibrium constant expression for each of the following:

(a)HC 2 H 3 O 2 + H 2 O −→ H 3 O + C 2 H 3 O 2 − (b)2 H 2 O −→ H 3 O + OH −
+
+
←−
←−
+
[H 3 O ][C 2 H 3 O 2 ]
−
Ans. (a) K a = (b) K w = [H 3 O ][OH ]
−
+
[HC 2 H 3 O 2 ]
17.17. Calculate the pH of each of the following solutions: (a) 0.0100 M HCl, (b) 0.0100 M NaOH,
(c) 0.0100 M Ba(OH) 2 , and (d) 0.0100 M NaCl.
Ans. (a) 2.000 (b) 12.000 (c) 12.301 (d) 7.000