Question

What is the percent by mass of a solution that contains 45.0 g `"KNO"_3` in 295 mL water?

Answer 1

The solution is 13.3 % `"KNO"_3` by mass.

Explanation:

The formula for percent by mass is

`color(blue)(bar(ul(|color(white)(a/a)"% by mass" = "mass of component"/"total mass" × 100 %color(white)(a/a)|)))" "`

We know the mass of the `"KNO"_3`, but we don't know the mass of the water.

Calculate the mass of the water

`"Mass of water" = 295 color(red)(cancel(color(black)("mL water"))) × "0.997 g water"/(1 color(red)(cancel(color(black)("mL water")))) = "294.1 g water"`

Now we can calculate the percent by mass.

`"Total mass = 45.0 g + 294.1 g = 339.1 g"`

`"% by mass" = (45.0 color(red)(cancel(color(black)("g"))))/(339.1 color(red)(cancel(color(black)("g"))))× 100 % = 13.3 %`