Exercise 5.1
A common talcum powder contains % m/m zinc oxide, which acts as an antiseptic. How much zinc oxide is present in g of the talcum powder?
NCERT’s answer
$\displaystyle 12$ g
$\displaystyle 12$ g of zinc oxide is present in $\displaystyle 300$ g of the talcum powder.
$\displaystyle 4$ % m/m means $\displaystyle 4$ g of zinc oxide in every $\displaystyle 100$ g of powder.
\(\displaystyle \text{Mass by mass percentage} = \dfrac{\text{Mass of solute}}{\text{Mass of solution}} \times 100 \)
\(\displaystyle 4 = \dfrac{\text{Mass of ZnO}}{300\ \text{g}} \times 100 \)
\(\displaystyle \text{Mass of ZnO} = \dfrac{4 \times 300}{100} = 12\ \text{g} \)