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# The density of NaOH solution is

Question:

The density of $\mathrm{NaOH}$ solution is $1.2 \mathrm{~g} \mathrm{~cm}^{-3}$. The molality of this solution is________ $\mathrm{m}$.

(Round off to the Nearest Integer)

[Use : Atomic masses : $\mathrm{Na}: 23.0 \mathrm{u} \quad \mathrm{O}: 16.0 \mathrm{u}$ $\mathrm{H}: 1.0 \mathrm{u}$

Density of $\mathrm{H}_{2} \mathrm{O}: 1.0 \mathrm{~g} \mathrm{~cm}^{-3}$ ]

Solution:

Consider $1 \ell$ solution

mass of solution $=(1.2 \times 1000) \mathrm{g}$

$=1200 \mathrm{gm}$

Neglecting volume of $\mathrm{NaOH}$

Mass of water $=1000 \mathrm{gm}$

$\Rightarrow$ Mass of $\mathrm{NaOH}=(1200-1000) \mathrm{gm}$

$=200 \mathrm{gm}$

$\Rightarrow$ Moles of $\mathrm{NaOH}=\frac{200 \mathrm{~g}}{50 \mathrm{~g} / \mathrm{mol}}=5 \mathrm{~mol}$

$\Rightarrow$ molality $=\frac{5 \mathrm{~mol}}{1 \mathrm{~kg}}=5 \mathrm{~m}$