A buffer solution is formed by mixing equal volumes of ${\text{1.00 mol}}\,{\text{d}}{{\text{m}}^{ - 3}}$ propanoic acid and ${\text{0.500 mol}}\,{\text{d}}{{\text{m}}^{ - 3}}$ potassium propanoate.

What is the concentration, in ${\text{mol}}\,{\text{d}}{{\text{m}}^{ - 3}}$, of ${\text{[}}{{\text{H}}^ + }{\text{(aq)]}}$ in this buffer solution? (${K_{\text{a}}}$ for propanoic acid is $1.30 \times {10^{ - 5}}$.)

A.     $2.60 \times {10^{ - 5}}$

B.     $1.95 \times {10^{ - 5}}$

C.     $1.30 \times {10^{ - 5}}$

D.     $0.650 \times {10^{ - 5}}$

A

This proved to be answered correctly by fewest candidates (37.59%) with many choosing option D, reversing [HA] and ${\text{[}}{{\text{A}}^ - }{\text{]}}$.