The vapour pressure of benzene at a certain temperature is 640mm of Hg. A non volatile and non electrolyte solid weighing 2.175 g is added to 39.08 g of benzene. The vapour pressure of the solution is 600mm of Hg. What is the molecular mass of the solid substance?

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Solution

Step 1:
$W_{B}$ = 2.175 g
$M_{A}$ = 78 g
$W_{A}$ = 39 g
p' = 640
p = 600
$M_{B}$ = Molar Mass of Solute

Step 2:
$M subscript B equals fraction numerator w subscript B M subscript A over denominator w subscript A begin display style fraction numerator left parenthesis p degree minus p right parenthesis over denominator p degree end fraction end style end fraction space space space space space space equals space fraction numerator 2.175 space x space 78 over denominator 39 space x space begin display style fraction numerator left parenthesis 640 space minus space 600 right parenthesis over denominator 640 end fraction end style end fraction space space space space space space equals space fraction numerator 2.175 space x space 2 over denominator 40 end fraction x 640 space space space space space space equals 69.6 space almost equal to space 70 space g$
Hence, the molecular mass of the solid substance is 70 g.

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