Molarity
Molarity (M) is the most common measure of solution concentration, defined as the number of moles of solute dissolved per litre of solution. It is temperature-dependent because liquid volumes change with temperature. Molarity is widely used in titrations, reaction stoichiometry involving solutions, and the preparation of standard laboratory solutions.
Key Formula
M = n / V
LaTeX: M = \dfrac{n}{V}
| Symbol | Meaning | Unit |
|---|---|---|
| M | Molarity (molar concentration) | mol/L (M) |
| n | Moles of solute | mol |
| V | Volume of solution | L |
Worked Example
Problem
What is the molarity of a solution prepared by dissolving 58.44 g of NaCl (molar mass = 58.44 g/mol) in enough water to make 500 mL of solution?
Solution
Step 1: Convert mass to moles: n = 58.44 g / 58.44 g/mol = 1.000 mol NaCl. Step 2: Convert volume to litres: V = 500 mL / 1000 mL/L = 0.500 L. Step 3: Calculate molarity: M = n / V = 1.000 mol / 0.500 L = 2.00 mol/L.
Answer
Molarity = 2.00 M (mol/L)
Common Laboratory Solution Concentrations
| Solution | Molarity (M) | Moles Solute per Litre | Typical Use |
|---|---|---|---|
| Dilute HCl | 0.1 M | 0.1 mol | Titrations |
| Saline (physiological) | 0.154 M NaCl | 0.154 mol | Medical infusions |
| Concentrated H₂SO₄ | 18.0 M | 18.0 mol | Chemical synthesis |
| Acetic acid (vinegar ~5%) | 0.83 M | 0.83 mol | Food science |
| NaOH standard | 1.00 M | 1.00 mol | Acid-base titrations |
Interactive Tools
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Related Terms
Molality
Molality (m) is a measure of solution concentration expressed as the number of moles of solute per kilogram of solvent (not solution). Unlike molarity, molality is independent of temperature and pressure because it is based on mass rather than volume, making it ideal for studying colligative properties such as boiling point elevation and freezing point depression. It is the preferred concentration unit in thermodynamic calculations involving solutions.
Mole Fraction
Mole fraction (χ) is a dimensionless concentration unit expressing the ratio of the moles of one component to the total moles of all components in a mixture. The sum of all mole fractions in a mixture always equals exactly 1. Mole fraction is used in Raoult's law, Dalton's law of partial pressures, and chemical thermodynamics because it is independent of temperature and pressure.
Molar Mass
Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol), and numerically equal to the substance's relative atomic or molecular mass in unified atomic mass units. It is calculated by summing the atomic masses of all atoms in one formula unit of the substance, using values from the periodic table. Molar mass is the essential conversion factor between grams (measurable in the lab) and moles (used in chemical calculations).
From Latin "moles" (mass, pile) combined with the suffix "-arity" indicating a ratio. The term became standard in chemistry textbooks in the late 19th century alongside the systematic development of solution chemistry.