Question

What is the percent by mass of KBr if 58.2 grams are dissolved in 344 grams of water?

Answer 1

`14.5%`

Explanation:

The idea here is that you need to determine how many grams of potassium bromide will be present for every `"100. g"` of this solution, which is what the solution's percent concentration by mass represents.

If you dissolve `"58.2 g"` of potassium bromide in `"344 g"` of water, the total mass of the solution will be

`"58.2 g + 344 g = 402.2 g"`

So this solution contains `"58.2 g"` of potassium bromide in `"402.2 g"` of the solution, which means that in `"100. g"` of this solution you will have

`100. color(red)(cancel(color(black)("g solution"))) * "58.2 g KBr"/(402.2 color(red)(cancel(color(black)("g solution")))) = "14.5 g KBr"`

You can thus say that the solution's percent concentration by mass is equal to

`color(darkgreen)(ul(color(black)("% m/m = 14.5% KBr")))`

The answer is rounded to three sig figs, the number of sig figs you have for your figures.

Answer 2

The percent by mass of `"KBr"` is `"14.4%"`.

Explanation:

`"%(m/m)"=("mass of solute")/("mass of solution")xx100`

The mass of the solution is the mass of the solute and solvent.

The given mass of the solute `"KBr"` is `"58.2 g"`.

The mass of the solution is `"58.2 g KBr + 344 g H"_2"O"="404.2 g"`

Plug in the known values and solve.

`"%(m/m)"=(58.2"g KBr")/(404.2"g solution")xx100=14.4%`