There are a number of ways to express concentration of a solution. This includes molarity and molality. Molarity is expressed as the number of moles of solute per volume of the solution while molality is expressed as the number of moles solute per mass of solution. Calculation is as follows:
(2 mol NaCl / L solution) ( 1 cm^3 / 1.35 g) ( 1000 L / 1 cm^3) (1000 g /kg) = 1.5 x 10^6 mol / kg solution