Some Basic Concepts of Chemistry - Result Question 11
11. A solution of sodium sulphate contains $92 g$ of $Na^{+}$ions per kilogram of water. The molality of $Na^{+}$ions in that solution in $mol kg^{-1}$ is
(2019 Main, 9 Jan I)
(a) 16
(b) 4
(c) 132
(d) 8
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Answer:
Correct Answer: 11. (b)
Solution:
Molality $(m)=\dfrac{\text { Number of moles of solute }}{\text { Mass of solvent (in g) }} \times 1000$
$\begin{aligned} & =\dfrac{\text { Mass of solute }(\text { in } g) \times 1000}{\begin{array}{r} \text { Molecular weight of solute } \times \text { mass of solvent }(\text { in } g) \end{array}} \\ & =\dfrac{w _{Na^{+}} \times 1000}{M _{Na^{+}} \times w _{H _2 O}}=\dfrac{92 \times 1000}{23 \times 1000}=4 mol kg^{-1} \end{aligned}$
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