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Question

# From relative lowering of vapour pressure , molecular mass of non-volatile solute can be determined. Which of the following relations is correct? Here, P∘A is vapour pressure of pure solvent PS is vapour pressure of solution wA,wB are mass of solvent and solute respectively MA,MB are molecular masses of solvent and solute respectively

A
MB=wB×MA×PSwA×(PAPS)
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B
MB=wA×MA×PSwB×(PAPS)
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C
MB=wB×(PAPS)wA×MA×PS
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D
None of the above
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Solution

## The correct option is A MB=wB×MA×PSwA×(P∘A−PS)Relative lowering of vapour pressure When the solution is not dilute (P∘A−PS)P∘A=nBnA+nB On reversing the equation: P∘AP∘A−PS=1+nAnB P∘AP∘A−PS−1=nAnB PSP∘A−PS=nAnB P∘A−PSPS=nBnA ⇒P∘A−PSPS=wB×MAwA×MB MB=wB×MA×PSwA×(P∘A−PS)

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