Question

How many grams of potassium chloride are in 45.0 g of a 5.00% (by mass) solution?

Answer 1

`"2.25 g KCl"`

Explanation:

A solution's percent concentration by mass, `"% m/m"`, essentially tells you how many grams of solute, which in your case is potassium chloride, `"KCl"`, you get per `"100 g"` of solution.

You're dealing with a `"5.00% m/m"` solution of potassium chloride, so right from the start you should be able to look at this solution and say that it contains `"5.00 g"` of solute for every `"100 g"` of solution.

So, if `"100 g"` of solution will contain `"5.00 g"` of solute, it follows that `"45.0 g"` of solution will contain

`45.0 color(red)(cancel(color(black)("g solution"))) * overbrace("5.00 g KCl"/(100color(red)(cancel(color(black)("g solution")))))^(color(purple)("= 5.00% m/m")) = color(green)(|bar(ul(color(white)(a/a)"2.25 g"color(white)(a/a)|)))`

The answer is rounded to three sig figs.