Question:
Calculate the mass of sodium sulphate required to prepare its $20 \%$ (mass percent) solution in $100 \mathrm{~g}$ of water?
Solution:
Mass percent $=\frac{\text { Mass of solute }}{\text { Mass of solute }+\text { Mass of water }} \times 100$
$20=\frac{x}{(x+100)} \times 100$
$20 x+2000=100 x$
$80 x=2000$ or $x=\frac{2000}{80}=25 \mathrm{~g}$
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