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4.375 g of a substance when dissolved in...

`4.375 g` of a substance when dissolved in `36.0 g` of water, lowered its vapour pressure by `0.5 mm` at a given temperature. The vapour pressure of water at this temperature is `25.0 mm`. Calculate the molecular weight of solute.

Text Solution

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`DeltaP = 0.5 mm`.using the formula for lowering of vapour pressure:
`DeltaP/P_(A)@=chi_(B)=n_(B)/(n_(A)+n_(B))=(W_(B)//Mw_(B))/(W_(A)//Mw_(A) + W_(B)//Mw_(B))`
`rArr 0.5/25 =(4.375//Mw_(B))/(36//18 + 4.375 //Mw_(B))`
`rArr Mw_(B) = 109.38 g mol^(-1)`
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