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🧪 Molarity Calculator

Calculate molarity (molar concentration, mol/L) from solute mass and solution volume, or calculate the required mass for a target molarity.

Calculation Mode

Solute Mass (g)

Target Molarity (mol/L)

Molar Mass (g/mol)

Solution Volume

What is this tool?

Molarity is the most widely used unit of concentration in chemistry, defined as the number of moles of solute dissolved per litre of solution, with the symbol M and the unit mol/L (often abbreviated simply as M, as in "a 0.5 M solution"). One mole contains exactly 6.022 × 10²³ entities (Avogadro's number), so a 1 molar solution contains one mole of solute particles in every litre of solution. The fundamental formula is M = n / V, where M is molarity, n is the amount of solute in moles, and V is the volume of the solution in litres. Because laboratory balances measure mass rather than moles, the number of moles is usually obtained from the measured mass using n = mass (g) / molar mass (g/mol). Combining the two relationships gives a single working equation: M = mass / (molar mass × V_in_litres). This molarity calculator implements that combined formula in both directions — you can either enter the mass, molar mass and volume to find the molarity, or enter a target molarity together with the molar mass and volume to back-calculate how much solid to weigh out. The tool also reports the number of moles and, in reverse mode, the required mass in grams, giving you all the quantities needed to prepare a solution of known concentration. Understanding molarity is essential for pH calculator work, buffer preparation and serial dilution calculator problems. If you need the molecular mass of a compound first, the molecular weight calculator can determine it from a chemical formula, and for radioactive decay problems involving concentration over time the half-life calculator is a natural companion. For related physical properties you may also find the density calculator useful when converting between mass and volume of pure liquids.

How it works

The calculator operates in two modes. In **Calculate Molarity** mode you enter the mass of the solute in grams, the molar mass of the compound in g/mol and the total solution volume; the tool converts volume to litres if needed, computes moles as n = mass / molar mass, and then returns molarity M = n / V_L. In **Calculate Mass** mode you enter the desired molarity, the molar mass and the volume; the tool reverses the equation to find the required mass: mass = M × molar mass × V_L. A unit toggle lets you specify the volume in millilitres or litres, and the calculator applies the factor 1000 automatically. The reference table below lists molar masses for five compounds commonly encountered in general-chemistry laboratories: | Compound | Formula | Molar Mass (g/mol) | Typical Use | |---|---|---|---| | Sodium chloride | NaCl | 58.44 | Physiological saline, titrations | | Sulphuric acid | H₂SO₄ | 98.08 | Battery acid, pH adjustment | | Sodium hydroxide | NaOH | 40.00 | Strong base, saponification | | Hydrochloric acid | HCl | 36.46 | Acid-base titrations, pH control | | Potassium permanganate | KMnO₄ | 158.03 | Redox titrations, disinfectant | For example, dissolving 5.844 g of NaCl (molar mass 58.44 g/mol) in 500 mL of water gives n = 5.844 / 58.44 = 0.100 mol and M = 0.100 / 0.500 = 0.200 M.

How to use

Select the calculation mode — find molarity from mass, or find mass from a target molarity.
Enter the molar mass of your compound in g/mol (see reference table for common values).
Enter the solution volume and choose millilitres or litres.
In molarity mode enter the solute mass in grams; in mass mode enter the desired molarity.
Click Calculate to see molarity, moles and (in mass mode) the grams of solute required.

Frequently Asked Questions

What is the difference between molarity and molality?

Molarity (M) is moles of solute per litre of solution, while molality (m) is moles of solute per kilogram of solvent. Molarity changes with temperature because solution volume expands or contracts, whereas molality is temperature-independent because it is based on solvent mass. For most routine bench work molarity is more convenient; for precise thermodynamic measurements molality is preferred.

How do I convert millilitres to litres for the molarity formula?

The formula M = n / V requires volume in litres. If your volume is given in millilitres, divide by 1000 to convert to litres (for example, 250 mL = 0.250 L). This calculator handles the conversion automatically when you select the mL unit — internally it divides by 1000 before applying the formula.

Can I use this calculator for liquid solutes like sulphuric acid?

Yes, but you must know the molar mass and enter the actual mass of the solute, not the volume pipetted. For concentrated acids the mass of pure solute differs from the mass of solution because of the water content; check the assay percentage on the reagent bottle. If you only know the volume and concentration of a stock solution, use the dilution calculator instead.

Why does my calculated molarity differ from the label on the bottle?

Common causes include temperature differences (molarity is volume-based and volume changes with temperature), evaporation of water from old stock, absorption of moisture by hygroscopic solids, and rounding of the molar mass. For titration-grade work always standardise the solution against a primary standard rather than relying on the calculated molarity alone.

What molar mass should I use for hydrated salts?

Use the molar mass of the full hydrated formula including water molecules. For example, copper(II) sulphate pentahydrate CuSO₄·5H₂O has a molar mass of 249.69 g/mol, not the 159.61 g/mol of the anhydrous salt. Entering the wrong value is a frequent source of error when preparing solutions from crystalline hydrates.

Is a 1 M solution the same as a 1 molal solution?

No. A 1 M solution contains 1 mole per litre of solution, while a 1 m (molal) solution contains 1 mole per kilogram of solvent. For dilute aqueous solutions the two values are numerically close because 1 L of water weighs about 1 kg, but they diverge for concentrated solutions or non-aqueous solvents with different densities.

Does this calculator account for activity coefficients?

No — this tool reports the nominal (stoichiometric) molarity based on the mass you enter. In concentrated or strong-electrolyte solutions the effective (active) concentration is lower than the nominal molarity because of ion pairing and interionic interactions. For pH calculations that require activity corrections, use the pH calculator after preparing the solution.

Disclaimer: This molarity calculator is intended for educational and laboratory-preparation purposes. Always verify prepared solutions against certified standards for analytical work, and consult your institution's chemical hygiene plan before handling strong acids, bases or toxic compounds.

Tips & Advice

Always dissolve the solute in a portion of solvent first and then bring the solution up to the final volume in a volumetric flask — adding the solid to the full volume of water can cause thermal contraction or expansion that throws off the concentration. Use analytical-grade reagents and a calibrated balance for work requiring high precision; even a small error in weighing propagates directly into the molarity. Remember that molarity is temperature-dependent because solution volume changes with temperature, so a solution prepared at 20 °C will have a slightly different molarity at 30 °C — for ultra-precise work consider molality instead. When dehydrated salts pick up moisture from the air the effective molar mass changes, so dry hygroscopic compounds (like NaOH) before weighing or use freshly opened reagent. For acid-base work, pair this calculator with the pH calculator to verify that the prepared concentration gives the expected hydrogen-ion activity, and use the dilution calculator when you need to prepare a lower-concentration working solution from a concentrated stock.

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