Question

A 3.0% (m/v) potassium chloride solution has a volume of 25.0 mL. How many moles of potassium chloride are in the sample?

Answer 1

Using your knowledge of m/v = g/mL, you can start by putting that in the equation as:

$\frac{3g}{100mL}$

And you know that the volume is 25 mL. So you could put this into the equation as:

$\frac{25mL}{1}$

And you also know that the molar mass of KCl is 74.55g (K = 39.1 g/mol and Cl = 35.45 g/mol. Add them together to get 74.55g/mol).

We can then put this in the equation as:

$\frac{1mol}{74.55g}$

Then, we can mulitply them all together:

$\frac{3g}{100mL}* \frac{25mL}{1} * \frac{1mol}{74.55g}=0.01 mol$

And now we know that there are 0.01 moles of KCl in the sample.