NH 3 + H 2 O → NH 4 + + OH−
According to the Brønsted-Lowry theory of acids and bases, an acid releases a proton while a
base accepts a proton. In the case of weak acids and bases, an equilibrium is established
whereby a weak acid, in this case H 2 O, dissociates partially, donating a proton to a weak base,
which is NH 3 . The weak acid, H 2 O, loses a proton and becomes a relatively stronger conjugate
base, OH−. This is one conjugate acid-base pair (H 2 O, OH−). Meanwhile, the weak base, NH 3 ,
picks up a proton to become a relatively stronger conjugate acid, NH 4 +. This is the second
conjugate acid-base pair (NH 4 +, NH 3 ).7 . A. NaOH is an Arrhenius base.
B. HCl is an Arrhenius acid and a Brønsted-Lowry acid.
C. NH 3 is a Brønsted-Lowry base and a Lewis base.
D. NH 4 + is an Arrhenius acid and a Brønsted-Lowry acid.
E. (CH 3 ) 3 N: acts only as a Lewis base.
8 . The dissociation of HA can be written as follows:
HA → H+ + A−
The molar ratio of A− to H+ is 1:1. We are told that at equilibrium [HA] is 0.94 M while [A−]
is 0.060 M. So [H+] must also be 0.060 M at equilibrium. It follows, then, that:A.
An acid with a high Ka is a strong acid because its equilibrium position lies farther to the
right, meaning that dissociation is more complete. The Ka of sulfurous acid is 1.7 × 10 −2
and the Ka of HA is 3.8 × 10 −3. The Ka of HA is less than that of sulfurous acid; therefore,
HA is a weaker acid.B.
C.
pH = −log [H+] = −log (0.060) = 1.22