Concentration

Concentration is the measure of the amount of solute in a certain amount of solvent. Knowing the concentration of a solution is important determining the strength of an acid or base (pH), among other things.  When there is so much solute present in a concentration that it no longer dissolves, the solution is saturated. 

Scientists often use molarity to measure concentration.

molarity = moles/Liter

Since reaction stoichiometry relies on molar ratios, molarity is the main measurement for concentration. 

A less common unit for concentration is called molality.

Molality = moles/Kg of solvent

Scientists sometimes use molality to measure concentration because liquid volumes change slightly based on the temperature and pressure. Mass, however, stays the same and can be measured accurately using a balance. Commercial concentrated products are usually expressed in mass percent; such as commercial concentrated sulfuric acid, which is 93-98% H₂SO₄ by mass in water (Hill, Petrucci 116).