**Question:**

The teacher instructed three students $A^{\prime}, B^{\prime}$ and ' $C$ ' respectively to prepare a $50 \%$ (mass by volume) solution of sodium hydroxide $(\mathrm{NaOH})$. 'A' dissolved $50 \mathrm{~g}$ of $\mathrm{NaOH}$ in $100 \mathrm{~mL}$ of water. ' $\mathrm{B}$ ' dissolved $50 \mathrm{~g}$ of $\mathrm{NaOH}$ in $100 \mathrm{~g}$ of water while ' $\mathrm{C}$ ' dissolved $50 \mathrm{~g}$ of $\mathrm{NaOH}$ in water to make $100 \mathrm{~mL}$ of solution. Which one of them has made the desired solution and why?

**Solution:**

The student ' $C$ ' has prepared the desired solution since it represents mass/volume solution. The student A' has taken into account volume of the solvent while the student ' $B$ ' has considered the mass of the solution

Mass by volume $\%=\frac{\text { Mass of solute }}{\text { Volume of solution }} \times 100=\frac{50}{100} \times 100=50 \%$ mass by volume.