Concentration - The amount of solute dissolved in a given amount of solvent or solution:

There are several ways to express concentration:

• Molarity (M) - The number of moles of solute per liter of solution expressed in mol/L = M:

• Mass percent - The percentage of the total mass of solute in a solution:

• Volume percent - The percentage of the total volume of solute in a solution, this is used for describing solutions of one liquid dissolved in another or a mixture of gases:

  e.g.  The alcohol content of liquor is expressed as a volume percent.

When it is important to know how much solute and solvent there is in a solution (instead of the amount of solution after the solute is added), the following two concentration units are used:

• Molality (m) - The number of moles of solute per kilogram of solvent:

NOTE: An advantage to using molality instead of molarity is that molality is not dependent on temperature whereas molarity is.  Molarity is expressed in terms of VOLUME of solution and the density of water (D = m/V) changes as the temperature changes.  Molality, however, is expressed in terms of MASS of solvent, so molality does not change with temperature.

• Mole fraction () - The mole fraction of any component is the total number of moles of solute and solvent that come from that component:

The mole fraction of the solute is the number of moles of solute divided by the total number of moles of solute and solvent.

The mole fraction of the solvent is the number of moles of solvent divided by the total number of moles of solute and solvent.

In a solution with a single solute dissolved in a solvent, the sum of the mole fractions of the solute and the solvent equals 1:

Next:  "Colligative Properties"