Discussion
The density of 3 M aqueous solution of a solute 'X' is 1.86 g mL-1 . The molality of the solution is (Molar mass of solute 'X' is 120 g mol-1 )
Answer: C
The molality is 2m.
Take 1 L of solution. Its mass is \(1000 \times 1.86 = 1860\) g, and it holds 3 mol of X, which weigh \(3 \times 120 = 360\) g. The solvent (water) is the rest:
\[ \begin{aligned} m_{\text{water}} &= 1860 - 360 = 1500\ \text{g} = 1.5\ \text{kg} \\ \text{molality} &= \frac{3}{1.5} = 2\ \text{mol kg}^{-1} \end{aligned} \]
3m confuses molality with molarity; 1m and the others come from forgetting to subtract the solute's mass or using the solution's volume as the solvent's mass.
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