The strengths of four acids are:

     glycine     ${\text{p}}{K_{\text{a}}} = {\text{9.87}}$

     chloroethanoic acid     ${K_{\text{a}}} = 1.38 \times {10^{ - 3}}$

     phenol     ${K_{\text{a}}} = 1.00 \times {10^{ - 10}}$

     butanoic acid     ${\text{p}}{K_{\text{a}}} = 4.82$

What is the order of increasing acid strength?

A.     ${\text{chloroethanoic acid}} < {\text{butanoic acid}} < {\text{phenol}} < {\text{glycine}}$

B.     ${\text{glycine}} < {\text{phenol}} < {\text{chloroethanoic acid}} < {\text{butanoic acid}}$

C.     ${\text{phenol}} < {\text{chloroethanoic acid}} < {\text{butanoic acid}} < {\text{glycine}}$

D.     ${\text{phenol}} < {\text{glycine}} < {\text{butanoic acid}} < {\text{chloroethanoic acid}}$

D

Candidates should be able to make an approximation of the inter-conversion of ${K_{\text{a}}}$ and ${\text{p}}{K_{\text{a}}}$ values. 69% of the candidates found no difficulty with this question.