Question

How many grams of `"KNO"_3` should be dissolved in water to make `"500.0 g"` of a `"20.0-ppm"` solution?

1) 1.00 x 10^-3 g
2) 1.00 x 10^-4 g
3) 1.00 x 10^-1 g
4) 1.00 x 10^-2 g

Answer 1

`1.00 * 10^(-2)` `"g"`

Explanation:

As you know, a solution's concentration in parts per million tells you the number of grams of solute present for every

`10^6 = 1,000,000`

grams of the solution. In your case, the target solution must have a concentration of `"20.0 ppm"`, which implies that it must contain `"20.0 g"` of potassium nitrate, the solute, for every `10^6` `"g"` of the solution.

This means that the mass of potassium nitrate needed to make this solution is equal to

`500.0 color(red)(cancel(color(black)("g solution"))) * "20.0 g KNO"_3/(10^6color(red)(cancel(color(black)("g solution")))) = color(darkgreen)(ul(color(black)(1.00 * 10^(-2) quad "g")))`

The answer is rounded to three sig figs.

So, if you dissolve `1.00 * 10^(-2)` `"g"` of potassium nitrate in enough water to get a total mass of the solution to `"500.0 g"`, you will have a `"20.0-ppm"` potassium nitrate solution.