Discussion
What is the mass of ethanoic acid required to prepare 0.5 m solution containing 100 g of be water? (Molar mass of ethanoic acid = 60 g mol-1).
Answer: A
The mass needed is 3 g.
Molality is moles of solute per kilogram of solvent. For 100 g (0.1 kg) of water at 0.5 m:
\[ \begin{aligned} n &= 0.5 \times 0.1 = 0.05\ \text{mol} \\ m &= 0.05 \times 60 = 3\ \text{g} \end{aligned} \]
6 g treats the solution as 0.5 mol per 200 g or doubles the moles, and 0.3 g uses 0.01 kg of water.
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