About the Molarity Calculator
Molarity is the concentration unit chemists use most in the lab. Recipes for buffers, titrations and reagent bottles are written in it, and almost every solution chemistry problem starts by converting between grams, moles, litres and mol/L.
This calculator handles both everyday tasks: working out the concentration of a solution you made, or how much solid to weigh out for a target concentration, and planning a dilution from a concentrated stock. It finds the molar mass from the formula automatically, using the same atomic weights as the molar mass calculator, so hydrates and bracketed formulas such as Ca(NO3)2 work as well.
What is molarity?
Molarity, symbol M, is the number of moles of solute dissolved per litre of solution: M = n/V. A 1 M sodium chloride solution contains one mole of NaCl, 58.44 g, in every litre of the final solution.
The volume is that of the whole solution, not of the solvent you started with. Dissolving a solid changes the volume slightly, which is why chemists dissolve the solute in some water first and then fill a volumetric flask up to the mark.
How to use the Molarity Calculator
- Choose molarity (M = n/V) or dilution (M₁V₁ = M₂V₂).
- Pick litres or millilitres for the volumes.
- For molarity, enter the formula if you want to work in grams.
- Leave exactly one quantity empty.
- Press Calculate and read the conversions and the check.
Formulas
Molarity
Moles from mass
Mass to weigh
Dilution
Millilitres to litres
Worked example: 0.5 mol in 250 mL
To prepare 500 mL of 0.1 M sodium chloride, first find the moles needed: mol.
The molar mass of NaCl is 58.44 g/mol, so weigh g, dissolve it in about 400 mL of water, then add water up to the 500 mL mark and mix.
The solution below is generated by the same engine that powers the calculator above.
Final answer
M = 2 mol/L
How we read your problem
Molarity is moles of solute per litre of solution.
Step-by-step solution
Convert the volume to litres
Molarity is moles per litre
Formulas used
Molarity
Moles from mass
Molarity (mol/L, written M) uses the volume of the whole solution, not of the solvent alone. To make a solution, weigh the solute, dissolve it in less solvent, then top up to the final volume.
Verification
- n = MV: 2 × 0.25 = 0.5 mol ✓
Detailed explanation
Moving between grams, moles and litres
Grams cannot go straight into M = n/V; they must first be converted to moles with the molar mass. That is the step most often skipped. Dissolving 10 g of NaOH (40.00 g/mol) in enough water to make 250 mL gives 0.25 mol in 0.25 L, a 1.0 M solution.
Volumes must be in litres. Typing 250 for a 250 mL flask while thinking in litres gives a concentration a thousand times too small, so the calculator lets you choose millilitres and converts for you, showing the step.
How dilution works
Adding solvent does not change the number of moles of solute; it only spreads them through a larger volume. Since n = MV before and after, M₁V₁ = M₂V₂. Both volumes just need the same unit, so millilitres work without conversion.
To make 500 mL of 1 M hydrochloric acid from 12 M stock, you need V₁ = (1 × 500)/12 ≈ 41.7 mL of stock, made up to 500 mL. For acids, always add the acid to water, never water to concentrated acid, because the heat released can make it spatter.
Molarity versus other concentration units
Molality, m, is moles of solute per kilogram of solvent. Unlike molarity it does not change with temperature, because mass does not expand when heated, so it is preferred for boiling-point and freezing-point calculations.
Mass percent and parts per million describe mass ratios and are common on product labels and in environmental data. For dilute water solutions, 1 ppm is about 1 mg per litre, which you can convert to molarity by dividing by the molar mass in mg/mol.
Common mistakes
- Using grams in M = n/V without converting to moles.
- Leaving the volume in millilitres when the formula needs litres.
- Using the solvent volume instead of the final solution volume.
- Mixing different volume units in M₁V₁ = M₂V₂.
- Writing the formula incorrectly, which changes the molar mass.
Tips
- Write units next to every number and cancel them.
- Check that a dilution gives M₂ smaller than M₁.
- For hydrated solids, include the water, e.g. CuSO4·5H2O.
- Round only the final answer, not intermediate values.
- Use the stoichiometry calculator for the next step of a reaction.
Frequently asked questions
How do I calculate molarity?
Divide the moles of solute by the volume of solution in litres: M = n/V. If you have grams, first divide by the molar mass to get moles.
How many grams do I need for a solution?
Multiply molarity by volume in litres to get moles, then by the molar mass: m = M × V × molar mass. Enter the formula and leave the grams empty.
What does 1 M mean?
One mole of solute per litre of solution. For glucose (180.16 g/mol) that is 180.16 g dissolved and made up to one litre.
How do I use M₁V₁ = M₂V₂?
Enter three of the four values in any one volume unit. The empty value is found by rearranging, and the calculator also tells you how much solvent to add.
What is the difference between molarity and molality?
Molarity is moles per litre of solution; molality is moles per kilogram of solvent. Molality does not change with temperature.
Does temperature affect molarity?
Slightly. Liquids expand when warmed, so the same moles occupy more volume and the molarity falls a little.
Can I use this for ions in solution?
Yes, for the formula unit. Remember that 1 M CaCl₂ gives 2 M chloride ions because each formula unit releases two.
Make up a solution
Choose molarity or dilution above and enter what you know.