9 . A higher Ka indicates a stronger acid; a lower Ka indicates a weaker acid. X is one-third the
value of 3X and therefore a weaker acid.
A.
[H+] is a direct measure of the strength of an acid. The greater the concentration of H+ in
solution, the stronger the acid. An acid that liberates X moles of H+ per liter is weaker,
therefore, than an acid that liberates 3X moles of H+ per liter.
B.
The lower the pH, the higher the concentration of H+. The acid with a pH of 3 X is thus the
weaker acid.
C.
10 . 1.82
If Kb = 2.22 × 10−11, then
The equilibrium expression for this dissociation is:
We can let [H+] = x at equilibrium and, since [H+]:[A−] = 1:1, [A−] = x.
If the original [HA] was 0.5 M, and x mol/L are dissociated, then at equilibrium, [HA] = 0.5 − x.
Thus the equilibrium expression becomes:
We can approximate that 0.5 − x ≈ 0.5 since HA has a small Ka, which indicates it is a weak acid.
11 . B
A buffer solution is prepared from a weak acid and its conjugate base, preferably in near-equal
quantities. Choices A and D are wrong because they do not show conjugate acid/base pairs.
