Equal masses of glucose (M = 180), urea (M = 60) and sucrose (M = 342) are dissolved in equal masses of water, forming three separate solutions. Which solution shows the **greatest** vapour pressure lowering?
AAll three give the same lowering
BUrea solution
CGlucose solution
DSucrose solution
Answer & Solution
Correct answer: B. Urea solution
$\Delta P \propto x_2 \approx n_2/n_1$, so for equal solvent mass the lowering scales with **moles** of solute, i.e. inversely with molar mass. Urea (smallest M = 60) gives the most moles per gram, hence the largest $\Delta P$. Sucrose (largest M) gives the smallest.
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