A student needs to prepare a buffer solution of propanoic acid and its sodium salt with pH 4 . The ratio of…

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

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

$\mathrm{pH}={\mathrm{pK}}_{\mathrm{a}}+\log \left(\frac{\left[{\mathrm{CH}}_3{\mathrm{COO}}^{-}\right]}{\left[{\mathrm{CH}}_3\mathrm{COOH}\right]}\right)$

$4=5-\log (1.3)+\log \left(\frac{\left[{\mathrm{CH}}_3{\mathrm{COO}}^{-}\right]}{\left[{\mathrm{CH}}_3\mathrm{COOH}\right]}\right)$

$\log \left(1.3\times {10}^{-1}\right)=\log \left(\frac{\left[{\mathrm{CH}}_3{\mathrm{COO}}^{-}\right]}{\left[{\mathrm{CH}}_3\mathrm{COOH}\right]}\right)$

$\frac{\left[{\mathrm{CH}}_3{\mathrm{COO}}^{-}\right]}{\left[{\mathrm{CH}}_3\mathrm{COOH}\right]}=0.13$