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.