Question

5.00g of a substance with molar mass 200 g have been dissolved in 50 g of a solvent with molar mass 60 g and vapour pressure 400 m. bar. find the vapour pressure of the solution
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Answer 1

Explanation:

The vapour pressure of the volatile material, i.e. the solvent, will be REDUCED proportional to the mole fraction of that solvent in the given solution…

χmole fraction of SOLUTE=5∙g200∙g∙mol−15∙g200∙g∙mol−1+50∙g60∙g∙mol−1=0.0291χmole fraction of SOLUTE=5•g200•g•mol−15•g200•g•mol−1+50•g60•g•mol−1=0.0291

χmole fraction of SOLVENT=50∙g60∙g∙mol−15∙g200∙g∙mol−1+50∙g60∙g∙mol−1=0.9709χmole fraction of SOLVENT=50•g60•g•mol−15•g200•g•mol−1+50•g60•g•mol−1=0.9709

And it is no mistake that the SUM of the individual mole fractions, χsolute+χsolvent=1χsolute+χsolvent=1 , i.e. as is required for a binary solution…

And the vapour pressure of the solution will be slightly reduced proportional to the NEW mole fraction of the solvent…

Psolution=χsolvent×400∙mm∙Hg=0.9709×400∙mm∙HgPsolution=χsolvent×400•mm•Hg=0.9709×400•mm•Hg .