Calculate the acid dissocation constant $K_a$ at $298\;K$ for a $0.010\;mol\;dm^{-3}$ solution of propanoic acid, $CH_3CH_2COOH$ The pH of this solution is $3.43$

Question

$\begin{array}{1 1} (a)\; 1.4 \times 10^{-3}\;mol\;dm^{-3}\\ (b) \;2.4 \times 10^{-3}\;mol\;dm^{-3} \\ (c) \;7.4 \times 10^{-3}\;mol\;dm^{-3} \\ (d) \;8.4 \times 10^{-3}\;mol\;dm^{-3} \end{array}$