What volume of $0.6\mathrm{M}$ sodium formate solution is required to prepare a buffer solution of $\mathrm{pH} 4.0$ by mixing it with $100 \mathrm{ml}$ of $0.8 \mathrm{M}$ formic acid. (Given: ${\mathrm{pK}}_{\mathrm{a}}$, for formic acid is $3.75$.)

Calculate the

i) hydrolysis constant,

ii) degree of hydrolysis and

iii) $\mathrm{pH}$ of $0.05 \mathrm{M}$ sodium carbonate solution $\left({\mathrm{pK}}_{\mathrm{a}}\right.$ for ${\mathrm{HCO}}_3^{-}$ is $10.26$).

Following solutions were prepared by mixing different volumes of $\mathrm{NaOH}$ of $\mathrm{HCl}$ different concentrations

(i) $60 \mathrm{mL} \frac{\mathrm{M}}{10} \mathrm{HCl} + 40\mathrm{mL} \frac{\mathrm{M}}{10} \mathrm{NaOH}$                 (ii) $55\mathrm{mL} \frac{\mathrm{M}}{10} \mathrm{HCl} + 45\mathrm{mL} \frac{\mathrm{M}}{10} \mathrm{NaOH}$

(iii) $75 \mathrm{mL} \frac{\mathrm{M}}{5} \mathrm{HCl} + 25\mathrm{mL} \frac{\mathrm{M}}{5} \mathrm{NaOH}$               (iv) $100 \mathrm{mL} \frac{\mathrm{M}}{10} \mathrm{HCl} + 100\mathrm{mL} \frac{\mathrm{M}}{10} \mathrm{NaOH}$

$\mathrm{pH}$ of which one of them is equal to $1$?