Why do we assume that it is 1 mol? A question in my book was as following:


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


In the answer, It assumed that the number of moles in the solution is 1 mole then subtracted 0.45 to get the number of moles of chloroform.

Why did he do that? Why specifically 1? It will obviously change if I assume any other number. (Unless I multiply the number of moles of benzine by that number)
Or should we just say that for every a certain amount of moles of that solution has a percentage and every other percentage is right if it follows the same instructions?
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.