Question

A buffer containing acetic acid and sodium acetate has a pH of 5.55. The K, value for CH3CO,H is 1.80 x 10. What is the ratio of the concentration of CH3CO H to CH3CO,t? [CH,CO,H1 CH2C0, 1 =

Answer 1

For acetic acid

$K_a=1.80 \times 10^{-5}$

$pK_a=-log_{10}K_a=-log_{10}1.80 \times 10^{-5}=4.74$

$pH=pK_a+log_{10} \frac{[CH_3CO_2^-]}{[CH_3CO_2H]}$

$5.55=4.74+log_{10} \frac{[CH_3CO_2^-]}{[CH_3CO_2H]}$

$5.55-4.74=log_{10} \frac{[CH_3CO_2^-]}{[CH_3CO_2H]}$

$log_{10} \frac{[CH_3CO_2^-]}{[CH_3CO_2H]}=0.81$

$\frac{[CH_3CO_2^-]}{[CH_3CO_2H]}=10^{0.81}=6.46$

Take reciprocal on both sides

$\frac{[CH_3CO_2H]}{[CH_3CO_2^-]} =\frac{1}{6.46}=0.155$