Why do we assume that it is 1 mol?
Sorry, but the accepted answer is wrong. The problem is indeterminate. Is there 1 liter of solution, or 5 liters, or 10 liters, or some other amount? 

If the problem was to be solved using mole fraction, then the problem should have been written that way. e.g.:


  Benzine and chloroform are considered very toxic organic solvents that dissolve in each other and in a solution of benzine and chloroform the mole fraction of benzine is 0.45. Find the the percent of mass( percent concentration solutions) in this solution.


There is no point in posing such sloppy problem statements.