For solutions, we use the idea of molarity (M), which is we state the number of moles present per liter of solution.
$$M = \frac{mol\,solute}{L\,solution}$$
So if we have 2 moles of something in 1.0 L of solvent, we would calculate:
$$\frac{2.0\,moles}{1.0\,Liters} = \frac{2.0\,moles}{L} = 2.0 M$$
A related unit, molality (m) is occasionally used in chemistry: it is moles of solute divided by the mass of solvent in kilograms. Since molality is not commonly used in the chemical sciences, we will not be addressing it further within this section [click here to learn more about molality] .
The following simulation explores the relation between concentration (as molarity) and the amounts of solute and solvent in the mixture: