Question

The concentration of urea in a solution prepared by dissolving 16 g of urea in 39 g of is __________% by mass. The molar mass of urea is 60.0 g/mol??

Answer 1

`29%`

Explanation:

You don't need to know the molar mass of urea to find the solution's percent concentration by mass, all you need is the mass of urea and the mass of water or the mass of the solution.

On that note, I presume that you're dissolving `"16 g"` of urea in `"39 g"` of water.

So, the idea here is that the solution's percent concentration by mass tells you the number of grams of urea, the solute, present for every `"100 g"` of the solution.

In your case, you get `"16 g"` of urea and `"39 g"` of water, so the total mass of the solution will be

`overbrace("16 g")^(color(blue)("mass of solute")) + overbrace("39 g")^(color(blue)("mass of solvent")) = overbrace("55 g")^(color(blue)("mass of solution"))`

So if you get `"16 g"` of urea in `"55 g"` of the solution, it follows that `"100 g"` of this solution will contain

`100 color(red)(cancel(color(black)("g solution"))) * "16 g urea"/(55color(red)(cancel(color(black)("g solution")))) = "29 g urea"`

This means that the solution's percent concentration by mass is equal to

`color(darkgreen)(ul(color(black)("% m/m = 29% urea")))`

This means that every `"100 g"` of this solution contain `"29 g"` of urea.

The answer is rounded to two sig figs.