Page 263 - Theory and Problems of BEGINNING CHEMISTRY

252 ACID-BASE THEORY [CHAP. 17

in pH. If OH is added to the solution, it reacts with H 3 O present. But the removal of that H 3 O is a stress,
+
−
+
which causes this equilibrium to shift to the right, replacing much of the H 3 O removed by the OH . The pH
−
+
does not rise nearly as much in the buffered solution as it would have in an unbuffered solution.
Calculations may be made as to how much the pH is changed by the addition of a strong acid or base. You
should first determine how much of each conjugate would be present if that strong acid or base reacted completely
with the weak acid or conjugate base originally present. Use the results as the initial values of concentrations for
the equilibrium calculations.
EXAMPLE 17.14. A solution containing 0.250 mol HC 2 H 3 O 2 and 0.150 mol NaC 2 H 3 O 2 in 1.00 L is treated with
0.010 mol NaOH. Assume that there is no change in the volume of the solution. (a) What was the original pH of the
−5
solution? (b) What is the pH of the solution after the addition of the NaOH? K a = 1.8 × 10 .
Ans. (a)
−→
HC 2 H 3 O 2 + H 2 O ←− C 2 H 3 O 2 + H 3 O +
−
[C 2 H 3 O 2 ][H 3 O ] −5
+
−
K a = = 1.8 × 10
[HC 2 H 3 O 2 ]
−→ + −
HC 2 H 3 O 2 + H 2 O ←− H 3 O + C 2 H 3 O 2
Initial 0.250 0 0.150
Change −x x x
Equilibrium 0.250 − x x 0.150 + x
Assuming x to be negligible when added to or subtracted from larger quantities, we find
(0.150)x
K a = = 1.8 × 10 −5
0.250
x = 3.0 × 10 −5
The assumption was valid.
pH = 4.52
(b) We assume that the 0.010 mol of NaOH reacted with 0.010 mol of HC 2 H 3 O 2 to produce 0.010 mol more of
C 2 H 3 O 2 (Sec. 10.3). That gives us
−
−→ + −
HC 2 H 3 O 2 + H 2 O ←− H 3 O + C 2 H 3 O 2
Initial 0.240 0 0.160
Change −x x x
Equilibrium 0.240 − x x 0.160 + x
(0.160)x
K a = = 1.8 × 10 −5
0.240
x = 2.7 × 10 −5
pH = 4.57

The pH has risen from 4.52 to 4.57 by the addition of 0.010 mol NaOH. (That much NaOH would have raised
the pH of 1.0 L of an unbuffered solution of HCl originally at the same pH to a final pH value of 12.00.)

Solved Problems

THE BRØNSTED-LOWRY THEORY
17.1. Explain the difference between the strength of an acid and its concentration.
Ans. Strength refers to the extent the acid will ionize in water. Concentration is a measure of the number of moles
of the acid in a certain volume of solution.