## Molarity, Molality, or Normality? (A Quick Review)

### Looking for a way to reinforce your students’ understanding of these concepts? Try this quick review.

**Molarity and Molality**

**Molarity (M)** is defined as the number of **moles** of solute per **liter** **of** **solution**.molarity = moles of solute/liters of solution

**Molality (m)** is defined as the number of **moles** of solute per **kilogram** **of** **solvent**.molality = moles of solute/kilograms of solvent

Although their spellings are similar, molarity and molality cannot be interchanged. **Molarity** is a measurement of the moles in the total volume of the solution, whereas **molality** is a measurement of the moles in relationship to the mass of the solvent.

When water is the solvent and the concentration of the solution is low, these differences can be negligible (d = 1.00 g/mL). However, when the density of the solvent is significantly different than 1 or the concentration of the solution is high, these changes become much more evident.

**Example:**Compare the molar and molal volumes of 1 mol of a solute dissolved in CCl_{4} (d = 1.59/mL).

For a **1 Molar** solution, 1 mol of solute is dissolved in CCl_{4} until the final volume of solution is 1 L.

For a **1 molal** solution, 1 mol of solute is dissolved in 1 kg of CCl_{4}.1 kg of CCl_{4} × (1,000 g/1 kg) × (mL/1.59 g) = 629 mL CCl_{4}

#### Normality

**Normality (N)** is defined as the number of **mole equivalents** per **liter** **of** **solution**:normality = number of mole equivalents/1 L of solution

Like molarity, normality relates the amount of solute to the total volume of solution; however, normality is specifically used for acids and bases.

**How to calculate normality from molarity**

The mole equivalents of an acid or base are calculated by determining the number of H+ or OH- ions per molecule: N = *n* × M (where *n* is an integer)

For an **acid solution**, *n* is the number of H+ ions provided by a formula unit of acid.**Example: **A 3 M H_{2}SO_{4} solution is the same as a 6 N H_{2}SO_{4} solution.

For a **basic solution**, *n* is the number of OH- ions provided by a formula unit of base.**Example: **A 1 M Ca(OH)_{2} solution is the same as a 2 N Ca(OH)_{2} solution.

**Note:** *The normality of a solution is NEVER less than its molarity!*

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