How much amount of water should be added to 25ml of 6M HNO₃  to prepare 0.5M HNO₃ solution ? 

$$\begin{gathered} {\mathrm{M}}_{\mathrm{i}}{\mathrm{V}}_{\mathrm{i}}={\mathrm{M}}_{\mathrm{f}}{\mathrm{V}}_{\mathrm{f}} \\ 6\times 25=0.5\times {\mathrm{V}}_{\mathrm{f}} \\ {\mathrm{V}}_{\mathrm{f}}=\frac{6\times 25}{0.5} \\ {\mathrm{V}}_{\mathrm{f}}=\frac{150}{0.5} \\ {\mathrm{V}}_{\mathrm{f}}=300\mathrm{ml} \end{gathered}$$

Volume of solvent(water) added $={\mathrm{V}}_{\mathrm{f}}-{\mathrm{V}}_{\mathrm{i}}=300-25=275\mathrm{ml}$