Question

The vapour pressure of pure benzene at certain temperature is 640 mm Hg.2h of a nine volatile solid is added to 39 g if benzene.The vapour pressure of the solution is 600mm.The molecular weight of the substance is ​

Answer 1

I am writing the steps so that it can help you.
first calculate moles of solute i.e 2/M here M is molecular weight of solute
then calculate moles of benzene i.e 39/78

then calculate mole fraction of solute i.e (moles of solute)/ total moles

relative lowering in vapour pressure is (640-600)/640
this value will be equal to the mole fraction of solute.
solve for M .