18 mL of mixture of acetic acid and sodium acetate required 6 mL of $0.1 \mathrm{M} \mathrm{NaOH}$ for neutralization of the acid and $12 \mathrm{mL}$of $0.1 \mathrm{M} \mathrm{HCl}$ for reaction with for reaction with salt, separately. If $p{K}_a$ of the acid is 4. 75, what is the pH of the mixture? $[\log 2=0.3]$