Question

How many moles of acetic acid and sodium acetate each should be dissolved to prepare one litre of $0.063$ molar buffer solution of pH $4.5$?
($K_a$ for ${CH}_{3}COOH=1.8\times {10}^{-5}$)

Answer 1

Applying Henderson's equation

$pH=\log \frac{\left[Salt\right]}{\left[Acid\right]}-\log K_a$

$\log \frac{\left[Salt\right]}{\left[Acid\right]}=4.5+\log 1.8\times {10}^{-5}=-0.2447$

$\left[Salt\right]=0.5692\times \left[Acid\right]$

$\frac{\left[Acid\right]}{\left[Salt\right]}=0.063$

$\left[Acid\right]=\frac{0.063}{1.5692}=0.040M$

no of moles $=n=0.04\times 1=0.04mol$

$\left[Salt\right]=(0.063-0.040)=0.023M$

no of moles $=n=0.023\times 1=0.023$