Calculate the vapour pressure of solution if relative lowering of vapour pressure and vapour pressure of pure solvent are 0.018 and 18 mm Hg respectively at 300 K.
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Always apply a logic check to your final answer: the addition of a non-volatile solute strictly lowers the vapour pressure. Therefore, your calculated $P_s$ MUST be smaller than the given $P^\circ$. Options (a) and (d) are immediately impossible because they are higher than 18.
Step 1: Understanding the Concept:
Relative Lowering of Vapour Pressure (RLVP) is defined as the ratio of the lowering of vapour pressure to the vapour pressure of the pure solvent: $RLVP = \frac{P^\circ - P_s}{P^\circ}$. Step 2: Formula Application:
$RLVP = 0.018$
$P^\circ = 18$ mm Hg
$P_s = ?$ Step 3: Explanation:
$0.018 = \frac{18 - P_s}{18}$
$0.018 \times 18 = 18 - P_s$
$0.324 = 18 - P_s$
$P_s = 18 - 0.324 = 17.676$ mm Hg.
Rounding to two decimal places, we get 17.68 mm Hg. Step 4: Final Answer:
The vapour pressure of the solution is 17.68 mm Hg.