Molarity


Using concentration expressed in percent by volume or percent by mass to solve stoichiometry problems is not simple. Therefore, in chemistry, concentration is usually expressed in molarity. Molarity M is defined as the number of moles of solute per liter of solution:

molarity, M =
number of moles of solute
volume of solution in liters

For example, a solution of salt in water containing 1.000 mol or 58.44 g NaCl in 1.000 L of solution is a 1.000 M solution. Molarity is a much more convenient unit of concentration for solving stoichiometry problems than are mass and volume percent, because it relates volume of solution, which is easily measured, to number of moles of solute.

Once a solution of a certain molarity is known, it can be used to measure known amounts of solute by measuring volume. Measuring the volume of a liquid is much easier and quicker than measuring mass.

A second method of preparing solutions of known molarity is to dilute a more concentrated solution. Dilution results in spreading a given quantity of solute throughout a larger solution volume. The quantity of solute does not change.

Sometimes the concentration of the concentrated solution is given in mass percent instead of in molarity. The concentration must be converted to molarity before the solution can be used to prepare a more dilute solution of known molarity. To convert concentration in mass percent to concentration in molarity, the density of the solution must be known.