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Question

The degree of dissociation of Ca(NO3)2 in a dilute aqueous solution containing 7g of the salt per 100g of water at 100C is 70%. If the vapour pressure of water at 100C is 760mm, calculate the vapour pressure of the solution.

A
378.5mm
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B
492.8mm
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C
746.10mm
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D
985.6mm
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Solution

The correct option is C 746.10mm
The molar mass calcium nitrate is 164 g/mol. 7 g of calcium nitrate 7164=0.0427 moles.

The molar mass of water is 18 g/mol. 100 g of water =10018=5.56 moles

The mole fraction of calcium nitrate =X=0.04270.0427+5.56=0.00762

The degree of dissociation α=70 % =0.7

The van't Hoff factor i=[1+(n1)α]
i=[1+(31)×0.7]
i=2.4

The relative lowering of the vapour pressure PoPPo=iX

760P760=2.4×0.00762=0.018
760P=13.9

P=746.10mm

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