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

May 16, 2016

$\text{2.25 g KCl}$

Explanation:

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

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

So, if $\text{100 g}$ of solution will contain $\text{5.00 g}$ of solute, it follows that $\text{45.0 g}$ of solution will contain

$45.0 \textcolor{red}{\cancel{\textcolor{b l a c k}{\text{g solution"))) * overbrace("5.00 g KCl"/(100color(red)(cancel(color(black)("g solution")))))^(color(purple)("= 5.00% m/m")) = color(green)(|bar(ul(color(white)(a/a)"2.25 g} \textcolor{w h i t e}{\frac{a}{a}} |}}}$

The answer is rounded to three sig figs.