Question

# The mass of a non-volatile solute of molar mass 40 g mol−1 that should be dissolved in 114 g of octane to lower its vapour pressure by 20% is :

A
10 g
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B
11.4 g
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C
9.8 g
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D
12.8 g
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Solution

## The correct option is C 10 gThe mass of a non-volatile solute of molar mass 40 g /mol that should be dissolved in 114 g of octane to lower its vapour pressure by 20% is 10 g.Let w g of the non-volatile solute is used. Its molar mass is 40 g/mol.The number of moles of non volatile solute =w40=0.025w moles.114 g of octane (molar mass 114 g/mol) corresponds to 114114=1 mole. The mole fraction of non volatile solute is =0.025w1+0.025w. The mole fraction of non volatile solute is proportional to the relative lowering in the vapour pressure.P0−PP0=XB20100=0.025w1+0.025 w100(0.025w)=20(1+0.025w)2.5w=20+0.50w2w=20w=10 gHence, the correct option is A

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