Question

A buffer solution is 0.20M in acetic acid and in sodium acetate. Calculate the change in pH up on adding 1.0ml of 0.10M HCl acid to 10 ml of this solution

Answer 1

The ICE approach for the equilibrium yields;

$CHCO_2H+H_2O \leftrightharpoons H_3O^{+}+CH_3CO_2^-$

The initial concentration of $CH_3CO_2H$ $=0.20$

Change $=x$

Equilibrium concentration$= 0.20-x$

The initial concentration of $CH_3CO_2^-$

.$=0.20$

Change$=x$

Equilibrium concentration$= 0.20+x$

Substituting the equilibrium concentration terms into the $K_a$ expression, assuming $x\lll$ $0.20$ and solving the simplified equation for $x$ yields

$x= 1.8\times 10^{-5}$

$[H_3O^+]=0+x=1.8\times 10^{-5}$ $pH=-log [H_3O^+]=-log (1.8\times 10^{-5}M)=4.74$