About the Molarity Calculator
This molarity calculator connects the mass of a solute, the volume of solution and its molar concentration (moles per litre). The most common lab question — “how many grams do I need to make 500 mL of 0.1 M NaCl?” — is the default, but you can also calculate the molarity of a solution you have already made, or the volume a given mass will make at a target concentration.
Type the solute’s chemical formula and its molar mass is calculated automatically from standard atomic weights, or enter the molar mass directly (for example from a reagent bottle, which is useful for hydrates and biomolecules). Concentrations can be in M, mM or µM and volumes in L, mL or µL.
Molarity is defined per litre of final solution, not per litre of solvent: dissolve the solute in less solvent than you need, then top up to the final volume in a volumetric flask.
How to use the molarity calculator
- 1Choose what you need: mass of solute, molarity or volume.
- 2Type the solute formula, or enter its molar mass directly.
- 3Enter the known values and pick units.
- 4Read the answer — for mass, weigh that amount and make up to the final volume.
Formula and method
Molarity is the number of moles of solute per litre of solution. The moles of solute come from its mass divided by its molar mass (MW). Combining the two gives the practical recipe: mass needed (g) = molarity (mol/L) × volume (L) × molar mass (g/mol).
The calculator converts mM and µM to mol/L, and mL and µL to litres, before applying the formula. The mass concentration in g/L is molarity × molar mass — handy when a protocol gives concentrations by weight.
- M
- Molarity (mol/L)
- n
- Moles of solute (mol)
- V
- Volume of solution (L)
- m
- Mass of solute (g)
- MW
- Molar mass of solute (g/mol)
Worked examples
500 mL of 0.1 M sodium chloride
You need 0.1 mol/L × 0.5 L = 0.05 mol of NaCl. At 58.44 g/mol that is 0.05 × 58.44 = 2.922 g.
Molarity of 20 g NaOH in 250 mL
NaOH is 22.990 + 15.999 + 1.008 = 39.997 g/mol, so 20 g is 0.50004 mol. Dividing by 0.25 L gives 2.00 M.
1 L of 50 mM glucose
50 mM is 0.05 mol/L. Glucose is 180.156 g/mol, so 1 L needs 0.05 × 180.156 = 9.008 g.
Frequently asked questions
What is molarity?+
Molarity (M) is the concentration of a solution in moles of solute per litre of solution. A 1 M NaCl solution contains 58.44 g of NaCl in every litre of final solution.
How do I calculate the grams needed for a solution?+
Multiply molarity (mol/L) by volume (L) by molar mass (g/mol). For 250 mL of 0.2 M KCl (74.55 g/mol): 0.2 × 0.25 × 74.55 = 3.73 g.
What is the difference between molarity and molality?+
Molarity is moles per litre of solution and changes slightly with temperature. Molality is moles per kilogram of solvent and does not depend on temperature.
How do I convert mM to M?+
Divide by 1,000. 250 mM equals 0.25 M, and 1 µM equals 0.001 mM or 0.000001 M.
Should I use the hydrate molar mass?+
Yes, if you are weighing a hydrated salt. For example CuSO4·5H2O is 249.68 g/mol versus 159.60 g/mol for anhydrous CuSO4; enter the formula with its water or type the molar mass from the bottle.