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Relative Lowering of Vapour Pressure

2.56 gm of su...

Question

2.56 gm of sulphur in 100 gm of CS $_{2}$ has depression in F.P. of 0.010 $^{o}$ C, K $_{f} =$ 0.1 $^{o}$ C m $^{- 1}$ . Calculate atomicity of sulphur in CS $_{2}$ :

8

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Solution

The correct option is A $8$
The expression for the depression in the freezing point and the molar mass of the solute is as shown below.
$Δ T_{f} = \frac{1000 \times K_{f} \times W_{2}}{M_{2} \times W_{1}}$
Substitute values in the above expression.
$0.010 = \frac{1000 \times 0.1 \times 2.56}{M_{2} \times 100}$
Thus the molecular weight of sulphur in $C S_{2}$ is $M_{2} = 0.256 k g / m o l = 256 g / m o l$
The atomic weight of the sulphur atom is 32 g/mol.
Thus atomicity of sulphur in $C S_{2}$ is $\frac{256}{32} = 8$ .

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