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How many grams of NaBr must be added to 270g of water to lower the vapour pressure by 3.125mm Hg at which vapour pressure of water is 50mm Hg (Na=23,Br=80) ? Assume 100% ionisation of NaBr.

Answer»

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SOLUTION :`NaBrhArrNa^(+)+Br^(-)`
van'tHoff FACTOR `i=2`
let moles of `NaBr=2n_(1)`
mole of `H_(2)O(mn_(2))=(270)/(18)=15`
`(DeltaP)/(P_("solvent")=X_("solute")=(n_(1)i)/(n_(1)i+n_(2))`
`(3.125)/(50)=(2n_(1))/(2n_(1)+15)`
`(2n_(1)+15)/(2n_(1))=(50)/(3.125)=16`
`1+(15)/(2n_(1))=16`
`(15)/(2n_(1))=15`
`n_(1)=0.5`
`therefore NaBr` taken `=0.5` mol `=0.5xx103=51.5g`.


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