A student needs to prepare a buffer solution of propanoic acid and...

Henderson's equation for calculating p^H of acidic buffer solutions is given as

$pH = {pK}_{a} + \log \left(\dfrac{\left[Salt\right]}{\left[Acid\right]}\right)$

$pH = {pK}_{a} + \log \left(\dfrac{\left[{CH}_{3}{COO}^{-}\right]}{\left[{CH}_{3}COOH\right]}\right)$

$4 = 5 - \log ( 1 . 3 ) + \log \left(\dfrac{\left[{CH}_{3}{COO}^{-}\right]}{\left[{CH}_{3}COOH\right]}\right)$

$\log \left(1 . 3 \times {10}^{- 1}\right) = \log \left(\dfrac{\left[{CH}_{3}{COO}^{-}\right]}{\left[{CH}_{3}COOH\right]}\right)$

$\dfrac{\left[{CH}_{3}{COO}^{-}\right]}{\left[{CH}_{3}COOH\right]} = 0 . 13$