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