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Question

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

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