The vapour pressure of ethanol and methanol are 44.5 mm and 88.7 mm Hg respectively. An ideal solution is prepared at the same temperature by mixing 60 g of ethanol with 40 g of methanol. The total vapour pressure of the solution is:
A
67.13 mm Hg
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B
66.13 mm Hg
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C
76.13 mm Hg
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D
None of the above
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Solution
The correct option is C 66.13 mm Hg Ethanol CH3CH2OH Mol. weight = 42 gms Mass added in the solution = 60 gms MoleCH3CH2OH=60/42=10/7
Methanol CH3OH Mol. weight = 28 gms Mass added in the solution = 40 gms MoleCH3OH=40/28=10/7
Mole Ratio nCH3CH2OH=MoleCH3CH2OHNtotal=0.5 nCH3OH=MoleCH3OHNtotal=0.5
Vapour Pressure Calculations
Vapor Pressure of Solution = nCH3CH2OH×PvapCH3CH2OH+nCH3OH×PvapCH3OH = 0.5×44.5+0.5×88.7 mm = 66.13 mm