Question

For an acid or a base, when is the normality of a solution equal to the molarity of the solution and when are the two concentration units different?

Short answer

The normality and molarity of an acid or base solution are equal when the solute produces one equivalent per mole, such as in HCl or NaOH. They are different when the solute produces more than one equivalent per mole, as in H2SO4 or Ca(OH)2. The relationship between normality and molarity is given by Normality = Molarity × n, where "n" is the number of equivalents per mole of solute.

Step-by-step solution

Define Normality

Normality (N) is a measure of concentration defined as the number of equivalent units of solute per liter of solution. An equivalent unit is the amount of substance that will react with or provide one mole of active species, such as hydrogen ions (H+) in an acid or hydroxide ions (OH-) in a base.

Define Molarity

Molarity (M) is another measure of concentration defined as the number of moles of solute per liter of solution.

Relationship between Normality and Molarity

The relationship between normality and molarity can be given as follows: Normality = Molarity × n where "n" is the number of equivalents per mole of solute in the solution. For example, in the case of an acid, "n" would be the number of hydrogen ions (H+) produced per mole of the acid. Similarly, for a base, "n" would be the number of hydroxide ions (OH-) produced per mole of the base.

When Normality and Molarity are equal

The normality and molarity of a solution are equal when the number of equivalents per mole of solute (n) is equal to 1. In other words, when an acid produces one H+ ion per mole, or when a base produces one OH- ion per mole, the normality and molarity are the same. Examples of such solutions are HCl (hydrochloric acid), which produces one H+ ion per mole, and NaOH (sodium hydroxide), which produces one OH- ion per mole.

When Normality and Molarity are different

The normality and molarity of a solution are different when the number of equivalents per mole of solute (n) is not equal to 1. In other words, when an acid produces more than one H+ ion per mole, or when a base produces more than one OH- ion per mole, the normality and molarity are different. Examples of such solutions are H2SO4 (sulfuric acid), which produces two H+ ions per mole, and Ca(OH)2 (calcium hydroxide), which produces two OH- ions per mole. In summary, normality and molarity of an acid or base solution are equal when the solute produces one equivalent per mole, and they are different when the solute produces more than one equivalent per mole.