Question

How many grams of KI are in 25.0 mL of a 3.0% (m/v) KI solution?

Answer 1

`"0.75 g"`

Explanation:

Notice that you were given the solution's mass by volume percent concentration, which is defined as mass of solute divided by volume of solution, and multiplied by `100`.

In your case, a `"3.0% w/v"` potassium iodide solution will contain `"3.0 g"` of potassium iodide for every `"100 mL"` of solution.

You know that the volume of the sample is four times smaller than this value, so you can say right from the start that it will contain four times less potassium iodide.

`25.0color(red)(cancel(color(black)("mL solution"))) * "3.0 g KI"/(100color(red)(cancel(color(black)("mL solution")))) = color(green)("0.75 g KI")`

The answer is rounded to two sig figs, the number of sig figs you have for the given concentration.