Molarity Calculator
Find the molarity of a solution, the mass of solute to weigh out, the volume to make up, or the molar mass. Type a formula such as NaCl and the molar mass is filled in, with preparation instructions.
How to Use
- Choose what to find: Molarity, Mass of solute, Volume or Molar mass.
- Say how the solute amount is given: as a mass (with its molar mass, or just a formula such as NaCl or CuSO4·5H2O) or directly in moles.
- Enter the other values with their units: g or mg, L or mL, M or mM.
- Or press a preset: 0.100 M NaCl, normal saline, 0.5 mol in 250 mL, or identifying a solute from its molar mass.
- Follow the preparation line under the buttons, and check Show Work for each step.
Worked Example
Making 1.000 L of 0.100 M NaCl. Moles needed: n = c × V = 0.100 × 1.000 = 0.100 mol. Mass: m = n × M = 0.100 × 58.44 = 5.844 g. Dissolve it in about 800 mL of water, transfer to a 1,000 mL volumetric flask and add water to the mark.
Finding a molarity. 9 g of glucose (180.156 g/mol) dissolved and made up to 500 mL: n = 9 ÷ 180.156 = 0.04996 mol; c = 0.04996 ÷ 0.500 = 0.0999 M (99.9 mM).
The common mistake: dividing by millilitres instead of litres. 0.5 mol in 250 mL is 0.5 ÷ 0.250 = 2 M, not 0.5 ÷ 250 = 0.002 M, a factor of 1,000 out. A close second is weighing the wrong form of a salt: 250 mL of 0.1 M copper sulfate needs 6.242 g of the pentahydrate (249.677 g/mol), not the 3.990 g you would use for the anhydrous powder.
Show Work
Formulas
Why Chemists Count in Moles per Litre
Reactions happen between particles, not grams: one sodium hydroxide neutralises one hydrochloric acid, whatever they weigh. Wilhelm Ostwald introduced the word “mole” in the 1890s for the amount of a substance whose mass in grams equals its molecular weight, and measuring liquids by volume made moles per litre the natural working unit. The mole became an SI base unit in 1971 and has been defined by the fixed Avogadro constant, 6.02214076 × 10²³ per mole, since 2019.
IUPAC’s formal name for molarity is “amount concentration”, written in mol/dm³ (the same as mol/L); the symbol M for mol/L is not an SI unit but is used everywhere in labs. Volumetric flasks are calibrated to contain their stated volume at 20 °C, which is why solutions are made up to the mark rather than mixed from measured volumes of solid and water.
The molar masses filled in from a formula use the IUPAC conventional atomic weights (H 1.008, C 12.011, O 15.999, Na 22.990, Cl 35.45), the same values as the Molar Mass Calculator.
About This Calculator
This calculator solves the molarity equation for whichever value you need: the concentration of a solution you have made, the mass of solid to weigh for a target concentration, the volume to make it up to, or the molar mass of an unknown solute. The solute can be entered as a mass, with its molar mass typed in or worked out from a formula, or directly as moles.
Every answer comes with plain preparation instructions, conversions between M, mM and g/L, a drawing of the flask, and the full working. Everything runs in your browser.
It is aimed at chemistry and biology students, lab technicians making up stock solutions and buffers, and anyone checking a concentration on a label.
Related tools: Molar Mass Calculator, Dilution Calculator, and Unit Converter.
Frequently Asked Questions
How do you calculate molarity?
Divide the moles of solute by the volume of solution in litres: c = n ÷ V. 0.5 mol in 250 mL is 0.5 ÷ 0.250 = 2 M. If you have a mass, turn it into moles first: 9 g of glucose (180.156 g/mol) is 0.04996 mol, and in 500 mL that is 0.0999 M.
How many grams do I need to make a solution?
Multiply molarity × volume in litres × molar mass: m = c × V × M. For 1.000 L of 0.100 M NaCl: 0.100 × 1.000 × 58.44 = 5.844 g. For 500 mL of 150 mM KCl: 0.150 × 0.500 × 74.548 = 5.591 g.
Why “make up to” the volume instead of adding the solid to 1 L of water?
Molarity is per litre of finished solution, and dissolving a solid changes the volume. Dissolve the 5.844 g of NaCl in about 800 mL of water, pour it into a 1,000 mL volumetric flask and add water up to the mark. Adding it to a full litre of water leaves you with slightly more than 1 L and a slightly weaker solution.
How do I convert between M, mM and g/L?
1 M = 1 mol/L = 1,000 mM. To get grams per litre, multiply by the molar mass: 0.154 M NaCl × 58.44 g/mol = 9.0 g/L, which is the 0.9% saline used in hospitals. Going the other way, a 5% glucose drip (50 g/L) is 50 ÷ 180.156 = 0.2775 M, or 277.5 mM.
What is the difference between molarity and molality?
Molarity (M) is moles per litre of solution; molality (m) is moles per kilogram of solvent. A 0.1 M solution has 0.1 mol in 1 L of finished solution; a 0.1 m solution has 0.1 mol dissolved in 1 kg of water. Molarity changes slightly with temperature because liquids expand; molality does not.
How do I use the Molarity Calculator?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Does it cost anything or need an account?
No. The tool is completely free, there is no account to create, and it keeps working offline after the page first loads.
Is anything I type uploaded?
No. The tool works entirely on your device, so the values you enter never leave your browser.
Common Use Cases
Lab stock solutions
0.100 M sodium chloride: weigh 5.844 g and make up to 1.000 L.
Biology buffers
500 mL of 150 mM KCl needs 5.591 g of potassium chloride.
Medical fluids
0.9% saline (9 g NaCl per litre) is 0.154 M, or 154 mM; 5% glucose is 277.5 mM.
Titration
250 mL of 1 M NaOH needs 0.25 × 39.997 = 10.00 g of sodium hydroxide.
Hydrated salts
250 mL of 0.1 M copper sulfate needs 6.242 g of the blue pentahydrate, CuSO4·5H2O, not 3.990 g.
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