The amounts of solute and solvent in a solution are expressed as concentration. The concentration value of a solution is the mass (commonly referred to as the weight) or volume of solute in a specified amount of solvent or total solution. There are three common methods of expressing concentration:

the per cent of solute by weight or by volume in 100 parts of solution. For example, an aqueous solution of 5 per cent sodium chloride by weight contains 5 grams of sodium chloride in 95 grams of water. Percentage is used to express the concentration of liquid, solid, or gaseous solutions.

the number of moles of solute per liter of total solution. (One mole of a substance is the amount whose weight in grams is numerically equal to the atomic weight or molecular weight.) For example, a 1 molar (1 M) aqueous solution of sodium chloride (molecular weight, about 58.44) contains one mole (in this instance, about 58.44 grams) of sodium chloride in enough water to make one liter of solution. Molarity is used to express the concentration of liquid solutions.

the number of moles of solute in 1,000 grams of solvent. For example, 1 molal (1 m) aqueous solution of sodium chloride contains one mole of sodium chloride in 1,000 grams of water. Molality, like molarity, is used to express the concentration of liquid solutions.