What it does
Molarity is the most common way to describe how concentrated a solution is: moles of solute per liter of solution. This calculator goes in any direction. Use it to find the molarity of a solution you made, the grams to weigh out for a recipe, or the volume a given mass will make. You can type the molar mass, or type the formula (like NaCl, CaCl2 or CuSO4·5H2O) and it's worked out from IUPAC standard atomic weights.
How it's calculated
Molarity (M) = n ÷ volume of solution (L)
mass = M × V × molar mass
1 M means 1 mole per liter (mol/L). IUPAC calls this amount concentration, and notes the symbol M is sometimes used for mol/L. The volume is the total volume of the finished solution, not the volume of water you started with.
Worked example
How much sodium chloride do you need for 250 mL of 0.5 M NaCl? The molar mass of NaCl is 22.990 + 35.45 = 58.44 g/mol. Moles needed = 0.5 mol/L × 0.250 L = 0.125 mol. Mass = 0.125 mol × 58.44 g/mol = 7.305 g. Dissolve it in less than 250 mL of water, then add water up to 250 mL.
Going the other way: 5.844 g of NaCl is 5.844 ÷ 58.44 = 0.1 mol. In 100 mL (0.1 L) of solution that's 0.1 ÷ 0.1 = 1 M.
Molar masses of common solutes
| Compound | Formula | Molar mass (g/mol) |
|---|---|---|
| Water | H₂O | 18.015 |
| Sodium chloride | NaCl | 58.44 |
| Sodium hydroxide | NaOH | 39.997 |
| Sulfuric acid | H₂SO₄ | 98.072 |
| Glucose | C₆H₁₂O₆ | 180.156 |
| Copper(II) sulfate pentahydrate | CuSO₄·5H₂O | 249.677 |
These are computed from the CIAAW abridged standard atomic weights. The molar mass calculator shows the breakdown for any formula.
Limitations
- Molarity changes slightly with temperature because liquids expand when warm. If that matters, use molality (moles per kilogram of solvent), which doesn't depend on volume.
- Hydrates count. If your bottle says CuSO4·5H2O, use that formula, because the water in the crystal is part of what you weigh.
- Purity: the answer assumes 100% pure solute. For a 98% reagent, divide the mass by 0.98.