The particle concentration within a solution describes the number of particles dissolved in the solvent. A solution may contain billions upon billions of particles, so chemists, for convenience, specify the amount of solute in terms of moles. Each mole contains 6.022 × 10^23 particles, and the mass of a mole of particles is the sum of the atomic weights of its elements. To find the concentration of a solution, you need to know the formula and mass of its solute.
Calculate the solute's formula mass by multiplying each of its elements' atomic weights by the number of that element's atoms in the solute. A mole of potassium chloride (KCl), for instance, has 1 mole of potassium, which has an atomic weight of 39.10, and 1 mole of chlorine, which has an atomic weight of 35.45: (1 × 39.10) + (1 × 35.45) = 74.55 grams per mole.
Divide the mass of solute in the solution its formula mass. If, for instance, the solution contains 100 g of potassium chloride -- 100 ÷ 74.55 = 1.32 moles.
Divide the number of moles by the volume of solution in liters (L). If, for instance, the solution is 1.5 L -- 1.32 ÷ 1.5 = 0.88. This is the solution's particle concentration, measured in molarity (M), or moles per liter.