Benzene and toluene form nearly ideal solutions . At 20 C , the vapour pressure of benzene is 75 torr and that of toluene is 22 torr . the partial vapour pressure of benzene at 20 C for a solution containing 78 gm benzene and 46 gm of toluene in torr is ?

Dear student

First of all the solution can be considered as an ideal solution there by stating that 
Psolution = P benzene + P toluene
Now P⁰ benzene = 75 torr
also P benzene = P^o benzene * mole fraction of benzene
Mole fraction = (given number of moles/total number of moles)
Moles of benzene = 1 mol = 78 g
moles of toluene = 46g/92 = 0.5 moles
so the total number of moles is 1+0.5 =1.5 mol
Thus, mole fraction is 1/1.5
So the 
P benzene = P^o benzene * mole fraction of benzene
P benzene = 75*(1/1.5) = 50 torr

Regards