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Chemistry · 2025 · 3 marks
CBSE 2025 · Region 2 · Set 1 · Q22
At 25 °C the saturated vapour pressure of water is 24 mm Hg. Find the saturated vapour pressure of a 5% aqueous solution of urea at the same…
At $\displaystyle 25$ °C the saturated vapour pressure of water is $\displaystyle 24$ mm Hg. Find the saturated vapour pressure of a $\displaystyle 5$% aqueous solution of urea at the same temperature. (Molar mass of urea $\displaystyle =60 \mathrm{~g} \mathrm{~mol}^{-1}$ )
Marking-scheme solution
$$\frac{p_1^0 - p_1}{p_1^0} = \frac{w_2 \times M_1}{M_2 \times w_1}$$$$\frac{24 - p_1}{24} = \frac{5 \times 18}{60 \times 95}$$$$\frac{24 - p_1}{24} = \frac{3}{190}$$$$24 - p_1 = \frac{3 \times 24}{190}$$$$p_1 = 23.62 \text{ mm Hg} \;/\; 23.64 \text{ mm Hg}$$
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CBSE Class 12 Chemistry past-paper question from the 2025 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.