Tamil Nadu Board Solutions for Chapter: Ionic Equilibrium, Exercise 9: Evaluate yourself - 9

How can you prepare a buffer solution of $\mathrm{pH} 9$. You are provided with $0.1\mathrm{M} {\mathrm{NH}}_4\mathrm{OH}$ solution and ammonium chloride crystals. (Given: ${\mathrm{pK}}_{\mathrm{b}}$ for ${\mathrm{NH}}_4\mathrm{OH}$ is $4.7$ at $25°\mathrm{C}$.

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 $3.75$.)