CBSE Class 12 · 2020 · SolutionsShort Answer

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Visha took two aqueous solutions — one containing 7.5 g of urea (molar mass = 60 g/mol) and the other containing 42.75 g of substance Z, each in 100 g of water. It was observed that both the solutions froze at the same temperature. Calculate the molar mass of Z.

Answer: M(Z) = 342 g mol⁻¹

Solution

Same freezing point means the same

Δ T_{f}

, hence the same molality.
Molality of the urea solution:

m = \frac{7.5/60}{0.100} = 1.25 mol kg^{- 1}

So moles of Z in 100 g water =

1.25 \times 0.100 = 0.125

mol.
Molar mass of Z:

M = \frac{42.75}{0.125}

M = 342 g mol^{- 1}

CBSE Class 12 Board · 2020 · Subjective

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