Concentration Calculator
Molarity, molality, and normality from mass and a chemical formula.
Molar mass of NaCl = 58.44 g/mol
Molarity is moles of solute ÷ liters of solution. Dissolving 5.85 g of NaCl (molar mass 58.44 g/mol) to make 0.500 L gives 5.85 ÷ 58.44 = 0.100 mol, then 0.100 ÷ 0.500 = 0.200 M. Molality uses kilograms of solvent; normality scales molarity by equivalents.
Three ways to measure concentration
Molarity (M) is moles of solute per liter of solution — the everyday lab unit. Molality (m) is moles per kilogram of solvent and is temperature-independent, making it the choice for colligative properties. Normality (N) scales molarity by the number of reactive equivalents per formula unit.
moles = mass (g) ÷ molar mass (g/mol)
Worked example
Find the molarity of 5.85 g of table salt (NaCl) dissolved to make 0.500 L of solution.
- 1 Find the molar mass of NaCl. Na 22.99 + Cl 35.45 = 58.44 g/mol.
- 2 Convert grams to moles. moles = 5.85 g ÷ 58.44 g/mol = 0.100 mol of NaCl.
- 3 Divide by the solution volume. molarity = 0.100 mol ÷ 0.500 L = 0.200 M.
Concentration units at a glance
Each divides the amount of solute by a different basis.
| Unit | Symbol | Definition |
|---|---|---|
| Molarity | M | moles of solute ÷ liters of solution |
| Molality | m | moles of solute ÷ kilograms of solvent |
| Normality | N | molarity × equivalents per formula |
| Mass percent | % w/w | grams solute ÷ grams solution × 100 |
| ppm | ppm | mg solute ÷ kg solution |
Choosing a unit and avoiding slips
Molarity vs molality. Use molarity for routine prep; use molality for freezing-point depression and boiling-point elevation, since it doesn’t change with temperature.
Solution vs solvent. Molarity uses the total solution volume after mixing — not the volume of solvent you started with. Molality uses the mass of solvent only.
Common mistakes. Dividing by milliliters instead of liters, forgetting to convert grams to moles, and treating equivalents as 1 for a diprotic acid like H₂SO₄ (it’s 2). Always derive moles from the molar mass first.