What is molarity?
Molarity (M) is the number of moles of solute per liter of solution: M = n / V. A 1 M solution of NaCl has 1 mol (58.44 g) of salt in every liter of solution. It is the most common way to express concentration in the lab.
Example · 5.85 g of NaCl in 250 mL
n = 5.85 / 58.44 = 0.1001 mol. V = 0.250 L. Molarity = 0.1001 / 0.250 = 0.400 M.
How to prepare a solution
To make a volume V of a solution with molarity M, you need m = M × V × molar mass grams of solute. For 500 mL of 0.1 M NaOH (40.00 g/mol): 0.1 × 0.5 × 40.00 = 2.00 g. Dissolve it in less water than you need and then top up to the mark in a volumetric flask.
Dilutions: C₁V₁ = C₂V₂
When you dilute, the moles of solute stay the same, so the initial concentration times the volume you take equals the final concentration times the final volume. To get 100 mL of 0.5 M from a 2 M stock: V₁ = 0.5 × 100 / 2 = 25 mL of stock, topped up to 100 mL.
Molality, normality, percent and ppm
- Molality (m) = moles of solute / kg of solvent. It does not depend on temperature.
- Normality (N) = molarity × equivalents per mole. 1 M H₂SO₄ is 2 N.
- Mass percent (w/w) = g of solute / g of solution × 100.
- Mass/volume percent (w/v) = g of solute / mL of solution × 100.
- ppm = mg of solute per liter of solution (for dilute aqueous solutions).
- Mole fraction (χ) = moles of solute / total moles.
The fourth tab gives all of them at once; it needs the density of the solution to convert volume into mass.
Frequently asked questions
Is molarity the same as molality?
No. Molarity is per liter of solution, molality per kilogram of solvent. For dilute aqueous solutions they are very close; for concentrated ones they differ.
What if my reagent is a hydrate?
Type the full formula (for example CuSO4·5H2O) so the molar mass includes the water of crystallization. For a detailed breakdown, use the molar mass calculator.