A student needs to prepare a buffer solution of propanoic acid and its sodium salt with pH 4.
The ratio of $\frac{\left[\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{COO}^{-}\right]}{\left[\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{COOH}\right]}$ required to make buffer is ……….
Given : $\mathrm{K}_{\mathrm{a}}\left(\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{COOH}\right)=1.3 \times 10^{-5}$