What Is Molarity and How Is It Calculated?
Molarity is a measure of solution concentration equal to the number of moles of dissolved solute per litre of total solution. Chemists write it as M, with units of mol/L, and use it to describe everything from lab reagents to intravenous saline concentrations.
The molarity formula is: M = mass (g) / (molar mass (g/mol) × volume (L)). Rearranged, this same relationship solves for mass — mass = M × molar mass × volume — or for volume — volume = mass / (M × molar mass).
The International Union of Pure and Applied Chemistry (IUPAC) defines molar concentration as an SI-derived quantity, formally expressed in mol/dm³, which is numerically identical to the more commonly used mol/L.
How to Use This Molarity Calculator
Choose which value you need — molarity, mass, or volume — then fill in the remaining two fields. The result updates instantly, with a step-by-step breakdown available underneath.
- Solve For: The unknown value you want the calculator to find.
- Solute: Pick a common compound to auto-fill its molar mass, or select Custom to enter your own.
- Molar Mass (g/mol): The mass of one mole of the compound — found on the periodic table or a certificate of analysis.
- Mass of Solute (g): The weight of dry compound dissolved into solution.
- Volume of Solution: The total final volume after dissolving, in litres or millilitres — not just the solvent added.
Molarity is a metric (SI-derived) unit, so this calculator works identically for laboratories anywhere in the world — there is no US/imperial equivalent to convert, unlike measurements such as length or weight.
Common Solute Molar Masses
Molar mass is the sum of the atomic weights of every atom in a compound's formula. The table below lists molar masses for compounds commonly prepared in teaching labs and clinical settings.
| Compound | Formula | Molar Mass (g/mol) |
|---|---|---|
| Sodium Chloride | NaCl | 58.44 |
| Sodium Hydroxide | NaOH | 40.00 |
| Hydrochloric Acid | HCl | 36.46 |
| Potassium Chloride | KCl | 74.55 |
| Glucose | C6H12O6 | 180.16 |
| Sucrose | C12H22O11 | 342.30 |
For example, standard 0.9% saline (normal saline) contains 9 grams of NaCl per litre of water, which works out to a molarity of 9 ÷ 58.44 ≈ 0.154 mol/L — the concentration considered isotonic with human blood plasma.
Frequently Asked Questions
What is a good example of molarity in everyday use?
Standard normal saline used in hospitals is 0.9% NaCl by weight, equal to 9 g of salt per litre of water, which calculates to approximately 0.154 mol/L — a concentration chosen because it matches the osmotic pressure of human blood.
How accurate is this molarity calculator?
This calculator applies the exact algebraic molarity relationship, so results are accurate to the precision of the molar mass and measurements you enter. Real-world lab accuracy also depends on balance precision, volumetric glassware calibration, and whether the compound is anhydrous or hydrated — a hydrate's water of crystallization adds to its molar mass.
What is the difference between molarity and molality?
Molarity (mol/L) measures moles of solute per litre of total solution, while molality (mol/kg) measures moles of solute per kilogram of solvent alone. Molality is preferred in experiments involving temperature changes because mass, unlike volume, does not expand or contract with heat.
How do I convert grams to molarity?
Divide the mass in grams by the compound's molar mass to get moles, then divide by the solution volume in litres: molarity = mass ÷ (molar mass × volume). This calculator performs that conversion automatically when you select "Molarity" as the value to solve for.