Explanation:
Given that,
Weight of $\mathrm{NaF}(\mathrm{w})=4.20 \mathrm{~g}$,
Weight of water $(\mathrm{W})=500 \mathrm{~g}$
Molecular weight of $\mathrm{NaF}=42 \mathrm{~g} / \mathrm{mol}$
Concentration $=$ ?
Now, Molality $=\frac{\mathrm{w} \times 1000}{\mathrm{M} \times \mathrm{W}}$Molality $=\frac{4.20 \times 1000}{42 \times 500}$
Molality $=0.200$ molal