M.C6H6 = 6x12+6x1 = 78 g/mol
M.C7H8 = 7x12+8x1 = 92 g/mol
m.p g 400g
C6H6 + CH3Cl --AlCl3--> C6H5-CH3 + HCl
78 92
=> m.p = 78x400/92 = 339,13 g benzen pur
100% ........................... 800 g benzen impur
p% .............................. 339,13 g benzen pur
= 42,39%