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Relative Lowering of Vapour Pressure

a Define the ...

Question

(a) Define the following terms:
(i) Mole fraction
(ii) Ideal solution
(b) 15.0 g of an unknown molecular material was dissolved in 450 g of water. The resulting solution was found to freeze at $- {0.34}^{o} C$ . What is the molar mass of this material? ( $K_{f}$ for water = 1.86 K kg mol $^{- 1}$ )

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Solution

(a) (i) The ratio of number of moles of a solute (components of a mixture) to the total number of moles in the mixture is called mole fraction $x_{A} = \frac{n_{A}}{n_{A} + n_{B}}$ or $x_{B} = \frac{n_{B}}{n_{A} + n_{B}}$
(ii) A solution that obey's Raoult's law or at all temperature and concentration is called an ideal solution.
Given : $W_{2} = 15.0 g, △ T_{f} = {0.34}^{o} C$
$W_{1} = 450 g, K_{f} = 1.86 K k g m o l^{- 1}$
(b) From the formula, $△ T_{f} = \frac{1000 \times K_{f} \times W_{2}}{W_{1} \times M_{2}}$
$M_{2} = \frac{1000 \times K_{f} \times W_{2}}{△ T_{f} \times W_{1}} = \frac{1000 \times 1.86 \times 15}{0.34 \times 450}$
$M_{2} = 182.35 g / m o l$

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