Question

Total Vapour Pressure of a solution mixing methyl alcohol and ethyl alcohol is represented by 254-119x in the unit of mm of Hg. Where X is mole fraction of ethyl alcohol. Find the vapour pressure of pure component

Answer 1

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Ptotal = pA + pB

Molecular weight of CH3OH = 12 + 3 + 16 + 1 = 32

Molecular weight of C­2H5OH = 24 + 5 + 16 + 1 = 46

According to Raoult’s law

Ptotal = p1 + p2

Where Ptotal = Total vapour pressure of the solution

P1 = Partial vapour pressure of one component

P2 = Partial vapour pressure of other componenet

Again, p1 = Vapour pressure (po1) * mole fraction

Similarly, p2 = Vapour pressure (po2) * mole fraction

Mole fraction of CH3OH = 40/32 /40/32+60/46 = 0.49

Mole fraction of ethanol = 60/46 /60/46+40/32 = 0.51

NOTE THIS STEP : Thus now let us first calculate the partial vapour pressures, i.e., p1 and p2 of the two component.

Partial vapour pressure of CH3OH(p1)

= 44.5 * 0.51 = 22.69 mm

∴ Total vapour pressure of the solution

= 43.48 + 22.69 mm = 66.17 mm

Mole fraction of CH3OH in vapour = 43.48/66.17 = 0.65

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