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 \({\text{[}}{{\text{A}}^ - }{\text{]}}\).