The \({\text{p}}{K_{\text{b}}}\) value of ammonia is 4.75 at 298 K. What is the \({\text{p}}{K_{\text{a}}}\) value of the ammonium ion?

A.     \(\frac{{{\text{1}}{{\text{0}}^{ - 14}}}}{{{\text{4.75}}}}\)

B.     \(\frac{{{\text{14.00}}}}{{{\text{4.75}}}}\)

C.     \(14.00 - 4.75\)

D.     \(\frac{{{\text{1}}{{\text{0}}^{ - 14}}}}{{{\text{1}}{{\text{0}}^{ - 4.75}}}}\)

C

We acknowledge the error in asking for the pKa of ammonia; it should, of course, have been the \({\text{p}}{K_{\text{a}}}\) of the ammonium ion. Nevertheless, over 79% of candidates chose the correct key (C) for answer.