# Question 865b5

Jan 18, 2018

Here's what I got.

#### Explanation:

In order to find the solution's percent concentration by mass, $\text{% m/m}$, you must find the number of grams of solute present for every $\text{100 g}$ of the solution.

You know that a sample of this solution contains $\text{5 g}$ of the solute dissolved in ${\text{50 mL = 50 cm}}^{3}$ of the solvent.

I will assume that the pure solvent has a density of ${\text{0.7 g cm"^(-3) = "0.7 g mL}}^{- 1}$, which means that the $\text{50-mL}$ sample has a mass of

50 color(red)(cancel(color(black)("mL"))) * "0.7 g"/(1color(red)(cancel(color(black)("mL")))) = "35 g"

So, the sample contains $\text{5 g}$ of the solute in

$\text{5 g + 35 g = 40 g}$

of the solution, which means that $\text{100 g}$ of this solution will contain

100 color(red)(cancel(color(black)("g solution"))) * "5 g solute"/(40color(red)(cancel(color(black)("g solution")))) = "12.5 g solute"#

Therefore, you can say that the solution's percent concentration by mass is equal to

$\textcolor{\mathrm{da} r k g r e e n}{\underline{\textcolor{b l a c k}{\text{% m/m = 12.5%}}}}$

I'll leave the answer rounded to three sig figs, but keep in mind that you should report the answer as

$\text{% m/m = 10%}$

because you have only one significant figure for your values.