Concentration Units

Concentration tells you how much solute is dissolved in a given amount of solution or solvent. There are four concentration units you need for the MCAT, and each is defined slightly differently. Knowing which one to use - and why - is the key.

Molarity (M) - Moles per Liter of Solution

Molarity is the most commonly used concentration unit in chemistry. It measures the number of moles of solute per liter of total solution.

Example: Dissolve 58.44 g of NaCl (1 mole) in enough water to make 500 mL of solution.

• M = 1 mol / 0.500 L = 2.0 M NaCl

Key limitation: Molarity changes with temperature because volume expands and contracts when heated or cooled. This is why colligative property equations use molality instead.

Molality (m) - Moles per Kilogram of Solvent

Molality measures the number of moles of solute per kilogram of solvent (not total solution).

Mole Fraction (X) - Ratio of Moles

Mole fraction is the ratio of moles of one component to the total moles of all components.

Example: A solution contains 2 mol of ethanol and 8 mol of water.

• $X_{\text{ethanol}}$ = 2 / (2 + 8) = 0.20
• $X_{\text{water}}$ = 8 / (2 + 8) = 0.80
• Check: 0.20 + 0.80 = 1.00 ✓

Normality (N) - Equivalents per Liter

Normality measures the number of equivalents per liter of solution. An equivalent is the amount of substance that reacts with or supplies one mole of the relevant species (H⁺, OH⁻, or electrons, depending on the reaction type).

Examples:

• HCl donates 1 H⁺: N = M x 1 (normality = molarity)
• H₂SO₄ donates 2 H⁺: N = M x 2 (1 M H₂SO₄ = 2 N)
• H₃PO₄ donates 3 H⁺: N = M x 3 (1 M H₃PO₄ = 3 N)

Comparison Table