Vapour pressure of pure benzene, = 70 Torr

Vapour pressure of pure methyl benzene, = 20 Torr

This mixture is equimolar, so number of moles of benzene,

n_A = number of moles of methyl benzene, n_B

Mole fraction of benzene in vapour phase, y_A = .... (i)

where p_A is pressure of benzene in mixture.



p_T is total pressure,

(pressure of methyl benzene)



Putting in above equation (i),



y_A = 0.78 = 78 10

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