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.