What is the difference between molarity and molality?

Molarity is moles of solute per litre of solution (mol/L) and changes slightly with temperature because volume does. Molality is moles per kilogram of solvent (mol/kg) and does not depend on volume.

Why must the final volume be the volume of solution, not of solvent?

Molarity is defined per litre of the finished solution. Dissolve the solute and make it up to the mark in a volumetric flask, rather than adding the solute to a measured volume of solvent.

Does the dilution formula work in other units?

Yes, as long as both concentrations use the same unit and both volumes use the same unit. C₁V₁ = C₂V₂ holds for mol/L, mM, mg/mL or percent.

Tools

Molarity and dilution calculator

Convert mass, moles and molarity, find how much to weigh, and plan dilutions with C₁V₁ = C₂V₂, all in your browser.

Solution from mass

Molar mass

g/mol

Or get molar mass from SMILES

Mass of solute

Final volume

Amount of substance: 0.025 mol
Molarity: 0.1 mol/L

Target molarity

mol/L

Mass to weigh: 4.504 g

Dilution (C₁V₁ = C₂V₂)

Stock concentration

mol/L

Final concentration

mol/L

Final volume

Stock to pipette: 10 mL
Solvent to add: 90 mL

Molarity is the amount of solute in moles per litre of solution: c = n / V, with n = m / M. This calculator works in both directions: from a weighed mass to the molarity, or from a target molarity to the mass you need. You can type the molar mass or get it from a SMILES string. The dilution section applies C₁V₁ = C₂V₂ to tell you how much stock to pipette and how much solvent to add. Everything runs locally in your browser.

How to calculate molarity and plan a dilution

1.Enter the molar mass in g/mol, or paste a SMILES string and press g/mol to calculate it.
2.Enter the mass of solute and the final volume of solution to get the moles and the molarity.
3.Or enter a target molarity to see the mass you need to weigh for that volume.
4.For a dilution, enter the stock concentration, the final concentration and the final volume to get the stock volume and solvent volume.

Worked example: 250 mL of 0.100 M aspirin, then a 1:10 dilution

1.Molar mass of aspirin (C9H8O4): 180.16 g/mol.
2.Moles needed: 0.100 mol/L × 0.250 L = 0.0250 mol.
3.Mass to weigh: 0.0250 mol × 180.16 g/mol = 4.504 g, made up to 250 mL in a volumetric flask.
4.To make 100 mL of 0.100 M from a 1.00 M stock: V₁ = (0.100 × 100 mL) / 1.00 = 10.0 mL of stock plus 90.0 mL of solvent.

Frequently asked questions

What is the difference between molarity and molality?: Molarity is moles of solute per litre of solution (mol/L) and changes slightly with temperature because volume does. Molality is moles per kilogram of solvent (mol/kg) and does not depend on volume.
Why must the final volume be the volume of solution, not of solvent?: Molarity is defined per litre of the finished solution. Dissolve the solute and make it up to the mark in a volumetric flask, rather than adding the solute to a measured volume of solvent.
Does the dilution formula work in other units?: Yes, as long as both concentrations use the same unit and both volumes use the same unit. C₁V₁ = C₂V₂ holds for mol/L, mM, mg/mL or percent.