Molar MassMolarity and making up solutions
Molarity is the usual way of stating the concentration of a solution in the laboratory. It is the number of moles of dissolved substance in one litre of solution. The unit is moles per litre, written mol/L or simply M.
molarity = moles of solute ÷ volume of solution in litres
A 1 M solution of sodium hydroxide contains 1 mole, 40.00 g, of sodium hydroxide in each litre of the finished solution.
Mass needed for a solution
mass in grams = molarity × volume in litres × molar mass
Example 1. Make 250 mL of 0.1 M sodium hydroxide. Volume = 0.250 L. Mass = 0.1 × 0.250 × 40.00 = 1.00 g.
Example 2. Make 500 mL of 0.05 M oxalic acid from the crystals, H2C2O4·2H2O. Mass = 0.05 × 0.500 × 126.06 = 3.15 g. Note that the molar mass of the dihydrate is used, because that is what is weighed.
Example 3. Make 1 litre of 0.02 M potassium permanganate. Mass = 0.02 × 1 × 158.03 = 3.16 g.
The right way to mix
- Weigh the solid accurately.
- Dissolve it in a beaker in less water than the final volume.
- Pour it into a volumetric flask and rinse the beaker into the flask.
- Add water up to the mark, then stopper and shake.
The solid is not added to a full litre of water. Molarity is per litre of solution, not per litre of water added. For acids, always add acid to water, slowly, never water to acid.
Finding molarity from a mass
Example 4. 5.85 g of sodium chloride is dissolved and made up to 500 mL. Moles = 5.85 ÷ 58.44 = 0.1001. Molarity = 0.1001 ÷ 0.500 = 0.200 M.
Dilution
Adding water does not change the number of moles of solute. So molarity × volume is the same before and after.
M1 × V1 = M2 × V2
Example 5. What volume of 2 M hydrochloric acid is needed to make 500 mL of 0.1 M acid? V1 = 0.1 × 500 ÷ 2 = 25 mL. Measure 25 mL of the 2 M acid and make it up to 500 mL with water.
Grams per litre
Concentration is sometimes given in grams per litre. To change to molarity, divide by the molar mass. A solution with 4.0 g of sodium hydroxide per litre is 4.0 ÷ 40.00 = 0.100 M. To go the other way, multiply.
Molarity and molality
Do not mix up molarity with molality. Molality is moles of solute per kilogram of solvent, written with a small m. It does not change with temperature, so it is used for boiling point and freezing point work. Molarity is used for titrations because volumes are easy to measure.
The converter includes a solution calculator: give it a formula, a molarity and a volume, and it tells you the mass to weigh.
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.