Molarity Calculator
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Enter any three of mass, molecular weight, volume and concentration and leave the fourth box blank. The molarity calculator converts between grams, moles, liters and concentration units such as M, mM, g/L and ppm.
Molarity, or molar concentration, is the number of moles of a dissolved substance per liter of solution. It is the most common way chemists and biologists express concentration, because chemical reactions happen between particles, and moles count particles: one mole is 6.02214076 × 10²³ of them, the Avogadro constant. A 1 M solution of anything contains the same number of solute particles per liter, regardless of how heavy those particles are.
Measuring molarity using molecular weight
In the lab you cannot count moles directly; you weigh the solute. The molecular weight, or molar mass, is the bridge. It is the mass in grams of one mole of the substance and is found by adding up the atomic weights of all the atoms in the formula. For sodium chloride, NaCl, that is 22.99 + 35.45 = 58.44 g/mol, so 58.44 grams of salt is one mole.
To prepare a solution of a target molarity, rearrange the formula: mass = molarity × volume × molecular weight. A liter of 0.5 M NaCl needs 0.5 × 1 × 58.44 = 29.22 g. Weigh the solid, dissolve it in less water than the final volume, then top up to the mark in a volumetric flask. Molarity refers to the volume of the finished solution, not the volume of water added, because dissolving a solid changes the total volume.
To dilute a stock solution, use C₁V₁ = C₂V₂: the moles of solute stay the same, so the concentration times the volume before dilution equals the concentration times the volume after. Making 500 mL of 0.1 M solution from a 2 M stock needs 0.1 × 500 ÷ 2 = 25 mL of stock made up to 500 mL.
Because volume expands and contracts with temperature, molarity changes slightly as a solution warms or cools. For work where that matters, chemists use molality instead.
Molarity (M) = moles of solute ÷ liters of solution = mass (g) ÷ [molecular weight (g/mol) × volume (L)]
Example: 100 g of NaCl (58.44 g/mol) in 2 L of solution is 1.711 mol ÷ 2 L ≈ 0.856 M.
Disambiguation of terminology
Several similar-sounding measures of concentration are used in chemistry, and mixing them up is a common source of error. Molarity and molality differ by a single letter but describe different things; in dilute water solutions their values are almost equal, while in concentrated solutions or other solvents they can differ a lot. Mass concentration and parts per million describe how much solute there is by mass rather than by number of particles. Normality, once common in titrations, depends on the reaction being carried out and is now discouraged by IUPAC in favor of molarity.
Molar mass and molecular weight are also used interchangeably in everyday lab work. Strictly, molar mass has units of g/mol, while molecular weight (relative molecular mass) is a dimensionless ratio to one-twelfth of the mass of a carbon-12 atom; numerically they are the same.
| Term | Definition | Typical unit |
|---|---|---|
| Molarity (c) | Moles of solute per liter of solution | mol/L (M) |
| Molality (b) | Moles of solute per kilogram of solvent | mol/kg (m) |
| Normality (N) | Equivalents of reactive units per liter of solution | eq/L |
| Mass concentration (ρ) | Mass of solute per volume of solution | g/L, mg/L |
| Mass fraction (w) | Mass of solute divided by total mass | %, ppm |
| Mole fraction (x) | Moles of one component divided by total moles | dimensionless |
| Molar mass (M) | Mass of one mole of a substance | g/mol |
Frequently asked questions
What is the formula for molarity?
Molarity = moles of solute ÷ liters of solution. If you have grams, divide by the molecular weight first: 100 g of NaCl (58.44 g/mol) is 1.711 mol, and in 2 L of solution that gives about 0.856 M.
What does 1 M mean?
One mole of solute per liter of solution. For glucose (180.16 g/mol), a 1 M solution contains 180.16 g of glucose in each liter.
What is the difference between molarity and molality?
Molarity is moles per liter of solution and changes slightly with temperature. Molality is moles per kilogram of solvent and does not.
How do I convert molarity to g/L?
Multiply the molarity by the molecular weight. 0.1 M NaCl is 0.1 × 58.44 = 5.844 g/L.
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