Using your knowledge of m/v = g/mL, you can start by putting that in the equation as:
[tex] \frac{3g}{100mL} [/tex]
And you know that the volume is 25 mL. So you could put this into the equation as:
[tex] \frac{25mL}{1} [/tex]
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:
[tex] \frac{1mol}{74.55g} [/tex]
Then, we can mulitply them all together:
[tex] \frac{3g}{100mL}* \frac{25mL}{1} * \frac{1mol}{74.55g}=0.01 mol [/tex]
And now we know that there are 0.01 moles of KCl in the sample.
