Molarity Calculator

Preparing a solution? Type the chemical formula and the calculator works out its molar mass, then finds the molarity from a mass and volume, the mass you need to weigh out for a target concentration, or the volumes for a dilution. It reads parentheses and hydrates, shows the working, and handles units from mg to kg and µL to L.

What molarity means

Molarity (M) is the number of moles of a solute dissolved in one litre of solution: M = moles ÷ litres. A 1 M sodium chloride solution contains one mole of NaCl — 58.44 g — in every litre of solution. Note that the volume is of the final solution, not the solvent added, so you dissolve the solute in less solvent and then make it up to the final volume in a volumetric flask.

Key formulas

How to make a solution of known molarity

1. Calculate the mass needed: concentration × volume in litres × molar mass.
2. Weigh the solute accurately on a balance.
3. Dissolve it in about two-thirds of the final volume of solvent in a beaker.
4. Transfer to a volumetric flask, rinsing the beaker into the flask.
5. Make up to the mark with solvent, with the bottom of the meniscus on the line.
6. Stopper and invert several times to mix; label with name, concentration, date and your initials.

Worked examples

• Mass needed: 250 mL of 0.5 M NaCl needs 0.5 × 0.250 × 58.44 = 7.305 g.
• Molarity: 7.305 g of NaCl in 250 mL gives 0.125 mol ÷ 0.250 L = 0.5 M.
• Dilution: to make 500 mL of 0.1 M from a 2 M stock, V1 = 0.1 × 500 ÷ 2 = 25 mL of stock, made up to 500 mL with water (a 20× dilution).
• Hydrates: copper(II) sulfate pentahydrate, CuSO4·5H2O, has a molar mass of about 249.7 g/mol — use this, not the anhydrous 159.6 g/mol, if weighing the blue crystals.

Entering formulas

Element symbols are case-sensitive: Co is cobalt, CO is carbon monoxide. Atomic masses are standard atomic weights; for isotopically labelled compounds, enter the molar mass directly.

Molarity, molality and percent

Molarity uses litres of solution and changes slightly with temperature because liquids expand. Molality (mol per kg of solvent) doesn’t, which is why it is used in physical chemistry for freezing-point and boiling-point calculations. Percent concentrations are also common: % w/v is grams per 100 mL, so a 0.9% saline is 9 g/L — about 0.154 M NaCl.

Serial dilutions

A serial dilution repeats the same dilution several times to reach very low concentrations accurately — for example, four 1:10 steps take a 1 M stock to 0.1 mM. Each step uses C1V1 = C2V2 with the previous step as the stock. Transferring 1 mL into 9 mL of diluent gives a 10-fold dilution; 1 mL into 1 mL gives 2-fold. The downloadable planner lays out each step with its transfer and diluent volumes.

Accuracy tips in the lab

• Use a volumetric flask, not a beaker, to make up the final volume.
• Weigh by difference for hygroscopic solids, and work quickly.
• Check whether your reagent is anhydrous or a hydrate — the label formula matters.
• Allow solutions that warm up on dissolving to cool before making up to the mark.
• For very small masses, make a more concentrated stock and dilute it rather than weighing tiny amounts.
• Record lot numbers and calculations on the preparation sheet.

Converting between units

Worked example with a hydrate

To make 100 mL of 0.1 M copper(II) sulfate from the blue pentahydrate crystals, type CuSO4·5H2O: the molar mass is about 249.68 g/mol. Moles needed = 0.1 mol/L × 0.100 L = 0.010 mol, so weigh out 2.497 g of crystals. Using the anhydrous molar mass (159.60 g/mol) by mistake would give 1.596 g — a solution about 36% too weak. Dissolve the crystals, make up to 100 mL in a volumetric flask and mix well.

Safety

Always add concentrated acid to water, never water to acid, and wear eye protection when handling chemicals. Check safety data sheets for the substances you use. Dilution calculations assume volumes add ideally; for concentrated solutions, make up to the final volume rather than adding a calculated amount of solvent.

Privacy

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