The number of equivalents per mole of solute depends on the reaction of interest. For acid-base reactions, (discussed in Acids and Bases) the molarity and normality are related by the number of protons an acid can donate. For monoprotic acids, like HCl, HF, and HClO4 the molarity and normality are equal. For diprotic acids like H2SO4 and H2C2O4 the normality is twice the molarity. For triprotic acids like H3PO4 the normality is three times the molarity. In redox reactions (discussed in Electrochemistry) the number of moles of electrons a molecule are ion can donate or accept determines the relationship between normality and molarity. For example, it is common for IO3- to give up five electrons. Therefore, the normality of a solution of IO3- is five times its molarity.

Molality is the number of moles of solute per kilogram of solvent and is abbreviated with a lower case m. The major advantage to using molality, m, instead of molarity, M, as a measure of concentration is that molality is temperature independent because it, unlike molarity, includes no volume term. As the temperature increase, the volume of solution generally increases slightly, causing a decrease in molarity but no change in molality. Therefore, if we are interested in the properties of a solution at different temperatures, as we will be when we discuss colligative properties, we should use molality. Due to 1 L of water having a mass of 1 kg (at 4oC), the molality and molarity of dilute aqueous solutions near room temperature are approximately the same value. The difference between molality and molarity becomes important for concentrated solutions or at temperatures much different than room temperature.

Another temperature independent measure of concentration is mass percent. Mass percent is defined as the mass of solute divided by the mass of the solution multiplied by 100%. Mass percents are useful when the molar mass of a compound, like a protein, is unknown.

The fifth and final measure of concentration we will discuss is called mole fraction. Mole fraction is the ratio of the number of moles of solute to the total number of moles of solution. This measure of concentration is particularly useful when talking about gaseous solutions and for some of the colligative properties.

To highlight the differences between those five measures of concentration, calculate the molarity, normality, molality, mass percent, and mole fraction of acetic acid, C2H3O2H, in a solution composed of 14.1 g of acetic acid and 250 g of water with a final solution volume of 260 mL. Compare your answers to the solutions given below:

To calculate the molarity, we find the number of moles of acetic acid, HAc, per liter of solution: