A molarity calculator finds the concentration of a solution in moles per liter from the moles of solute and the volume of solution. Formula: Molarity = moles / liters (moles >= 0, liters > 0). Displayed as M (mol/L). 0.25 mol in 0.5 L is 0.5 M.
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How to use the molarity calculator
- Enter the moles of solute.
- Enter the volume of solution in liters.
- Read the molarity in mol/L.
How the molarity calculator works
The molarity calculator divides the amount of solute in moles by the volume of the whole solution in liters. Molarity (M) = moles of solute ÷ liters of solution, so the unit is mol/L. Volume must be greater than zero. The result is shown to six decimal places, which is more precision than most lab measurements justify, so round it to match your data.
Molarity calculator worked example
Dissolve 0.25 mol of sodium chloride and add water until the solution measures 0.5 L. The molarity calculator gives 0.25 ÷ 0.5 = 0.5 M. If the same 0.25 mol were made up to 250 mL instead, convert to 0.25 L first and the answer becomes 1 M. The table shows more cases.
| Moles | Liters of solution | Molarity |
|---|---|---|
| 0.5 | 1.0 | 0.5 M |
| 0.25 | 0.5 | 0.5 M |
| 1.0 | 0.25 | 4 M |
| 0.1 | 2.0 | 0.05 M |
| 2.0 | 1.5 | 1.333333 M |
Starting from grams
Most recipes give a mass, not moles. Convert first: moles = grams ÷ molar mass. For example, 29.22 g of NaCl, with a molar mass of about 58.44 g/mol, is 0.5 mol. Made up to 1 L, that is a 0.5 M solution. A molecular weight tool can give the molar mass from the formula, and the two steps together answer most bench questions.
Use the volume of the solution
Molarity uses the final volume of the solution, not the volume of water you added. Dissolving a solid changes the volume, so the standard method is to dissolve the solute in some solvent in a volumetric flask and then fill to the mark. Volume also changes slightly with temperature, which is why molality, based on kilograms of solvent, is preferred when temperature varies.
Dilution with M1V1 = M2V2
When you dilute a stock solution, the moles of solute stay the same. So M1 × V1 = M2 × V2. To make 500 mL of 0.1 M from a 2 M stock, V1 = 0.1 × 0.5 ÷ 2 = 0.025 L, or 25 mL of stock made up to 500 mL. You can check the result with the molarity calculator: 0.05 mol in 0.5 L is 0.1 M.
Tips and common errors
- Convert milliliters to liters by dividing by 1,000.
- Moles cannot be negative; the tool rejects a negative number.
- For ionic compounds, the ion concentration can be a multiple of the formula concentration: 1 M CaCl2 gives 2 M chloride ions.
- Record significant figures from your balance and glassware, not from the screen.
Related concentration units
Millimolar (mM) is one thousandth of a mole per liter, so 0.005 M is 5 mM. Mass concentration in g/L equals molarity times molar mass. Percent solutions and parts per million describe mass or volume ratios instead, and they need density or molar mass to convert. Keep the unit with every number in your notebook.
Lab practice
Weigh the solute on a calibrated balance, transfer it fully into the flask, and rinse the weighing boat into the flask so nothing is lost. Dissolve, let the solution reach room temperature, then fill to the line with the bottom of the meniscus at eye level. Label the bottle with the compound, molarity, date and your initials. Strong acids and bases release heat when diluted, so add acid to water, never water to acid, and wear eye protection. These steps matter as much as the arithmetic when you need an accurate concentration.
For study and planning only. Check lab, engineering and safety work against your course, instrument or a qualified professional.
Related converters and calculators
Related calculators:
- Molecular weight calculator: molar mass
- Molar concentration converter: mM, µM and more
- Density calculator: mass and volume
- Percent error calculator: lab accuracy
Sources
OpenStax Chemistry 2e, 3.3: Molarity; OpenStax Chemistry 2e, 3.1: Formula mass and the mole concept.
Formula and method
Molarity = moles / liters (moles >= 0, liters > 0). Displayed as M (mol/L).
Direct concentration definition. Does not convert grams to moles; enter moles already, or convert with a molecular weight helper first.
Questions about this calculator
What is the molarity formula?
Molarity = moles of solute ÷ liters of solution.
How do I convert grams to moles?
Divide the mass by the molar mass in g/mol.
Do I use the water volume or the solution volume?
The final solution volume.
How do I dilute a stock?
Use M1V1 = M2V2.
What is 1 mM in M?
0.001 M.
Check the assumptions and units before relying on a result. Financial, tax and health estimates do not replace advice for your individual circumstances.
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