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Question

# 100 g of liquid A (molar mass 140 g molâˆ’1) was dissolved in 1000 g of liquid B (molar mass 180 g molâˆ’1) The vapour pressure of pure liquid B was found to be 500 torr. Calculate the vapour pressure of pure liquid A and its vapour pressure in the solution if the total vapour pressure of the solution is 475 torr:

A
3.19,500
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B
283.18,31.999
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C
500,300
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D
190,31.965
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Solution

## The correct option is D 283.18,31.999Given: PT=475,WA=100g,MA=140,PoB=500WB=1000g,MB=180,PoA?,PA=?PT=PoAXA+PoBXB⟶(1) & XA+XB=1⟶(2)⇒XBXA=WBMAWAMB=1000×140100×180=7.78∴XB=7.78XASubstituting XB in (2)XA+7.78XA=1∴XA=0.113 & hence XB=1−0.113=0.886Substituting XA & XB in equation (1)⇒475=PoA(0.113)+500(0.886)⇒0.113PoA=32⇒PoA=320.113=283.18torrNow, PA=PoA(1−XB)=PoAXA =283.18×0.113 =31.999torr

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