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Question

The vapour pressure of ethanol (A) and methanol (B) are 44.5mm Hg and 88.7mm Hg respectively. An ideal solution is formed at the same temperature by mixing 60 g of ethanol with 40 g of methanol. Calculate the total vapour pressure of the solution (PT) and the mole fraction of methanol in the vapour phase (YB)

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Solution

Molar mass of ethyl alcohol =46 g mol1
No. of moles of ethyl alcohol =6046=1.304 mol
Molar mass of methyl alcohol =32 g mol1
No. of moles of methyl alcohol =4032=1.25 mol
Mole fraction of ethyl alcohol, XA=1.3041.304+1.25=0.5107
Mole fraction of methyl alcohol, XB=1.251.304+1.25=0.4893
Partial pressure of ethyl alcohol =XA×PA=0.5107×44.5=22.73 mm Hg
Partial pressure of methyl alcohol =XB×PB=0.4893×88.7=43.40 mm Hg
Total vapour pressure of solution =22.73+43.40=66.13 mm Hg
Mole fraction of methyl alcohol in the vapour,YB=Partial pressure of CH3OHTotal vapour pressure=43.4066.13=0.6563

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