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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?

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    $\begingroup$ Can you update this question? "There is 0.45 mol" 0.45 mol of what? of total solution? 0.45 mol benzene? 0.45 mol chloroform? And what "percent of mass" do you want? Without knowing any of this, my guess is that 0.45 is the mole fraction of one of the solvents. Choosing the total number of moles to be 1 just makes the calculations easier, but you could choose any n and get the same result $\endgroup$ – Sean Doris Feb 26 '16 at 19:27
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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.

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This is the general assumption we take when we are short of information. We obviously couldn't solve the question without knowing the number of moles in the solution. There is no answer to "why specifically 1", it is generally assumed to be so. You are going to come across a lot of questions when you would require the initial number of moles. When the question maker would want you to take 1 mole,many a times he wouldn't even bother to mention that. You will be used to such assumptions as you will study more of physical chemistry.

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    $\begingroup$ The question is lacking some details, but in general, you can solve problems of this kind without stipulating any amount of moles. You can just call it $x$, and after doing calculations, you can find the ratio between the two substances in such a way that the $x$ cancels out. $\endgroup$ – Nicolau Saker Neto Feb 26 '16 at 21:20
  • $\begingroup$ The part where x cancel out didn't happen in my question. I kind of sure that it has to affect the concentration and it doesn't cancel out unless you have used another method. $\endgroup$ – Biker Feb 27 '16 at 17:35

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