Concentration formulas
| Term | Formula | Unit |
|---|---|---|
| Molarity (M) | moles of solute ÷ litres of solution | mol/L |
| Molality (m) | moles of solute ÷ kg of solvent | mol/kg |
| Normality (N) | Molarity × n-factor | eq/L |
| Mole fraction (x) | moles of one part ÷ total moles | none |
| Dilution | M₁V₁ = M₂V₂ | – |
Molarity vs molality. Molarity uses the volume of the whole solution, which changes with temperature. Molality uses the mass of the solvent only, so it doesn't. That's why colligative properties use molality.
A shortcut you'll use often
M = (mass in g × 1000) ÷ (molar mass × volume in mL). For 4 g of NaOH in 250 mL: (4 × 1000) ÷ (40 × 250) = 0.4 M.
To find the molar mass, use the molar mass calculator. For the pH of the solution, try the pH calculator.
Questions students ask
What is the molarity of 4 g NaOH in 250 mL of solution?
0.4 M. 4 g ÷ 40 g/mol = 0.1 mol, and 0.1 mol ÷ 0.25 L = 0.4 M.
How do I make 500 mL of 0.5 M HCl from 12 M stock?
V₁ = M₂V₂ ÷ M₁ = 0.5 × 500 ÷ 12 ≈ 20.8 mL of stock, made up to 500 mL with water. Always add acid to water.
What is the difference between molarity and normality?
Normality = molarity × n-factor. For H₂SO₄ (n-factor 2), 1 M = 2 N. For HCl, 1 M = 1 N.