Which of the following compounds should have the strongest conjugate acid? (See choices in answer).
The answer is indeed B. aniline.
The options are:
The strongest conjugate acid will correspond to the weakest base, which in your case is the base that has the smallest base dissociation constant,
For a generic weak base equilibrium, you have
#B_((aq)) + H_2O_((l)) rightleftharpoons BH_((aq))^(+) + OH_((aq))^(-)#
The base dissociation constant is defined as
The value of
If more molecules of base ionize to form
As a result, the value of
The stronger the base, the larger the concentrations of conjugate acid and hydroxide ions it produces in solution. An important consequence is that the equilibrium of stronger bases lies more to the right.
This implies that the reverse reaction
#BH_((aq))^(+) + OH_((aq))^(-) rightleftharpoons B_((aq))^(+) + H_2O_((l))#
is less likely to occur, meaning that the tendency of the base,
On the other hand, if the base is weak, the equilibrium will lie more to the left. This means that the conjugate acid will be more willing to donate its proton to reform the base
So, the smaller the value of
In your case, aniline will be weakest base, which implies that it will have the strongest conjugate acid.