Quick answer: To make 500 mL of 0.1 M sodium chloride (NaCl, 58.44 g/mol), weigh 0.1 × 0.5 × 58.44 = 2.922 g and make it up to 500 mL.
How it's calculated
Molarity (amount concentration) is moles of solute per litre of solution: c = n ÷ V, in mol/L, written M. Moles come from mass: n = m ÷ M, where M is the molar mass in g/mol.
Putting them together, the mass to weigh is m = c × V × M, and the molarity of a solution you made is c = m ÷ (M × V). Volumes in mL are divided by 1000 to give litres.
Enter the formula (NaCl, CuSO4·5H2O, C6H12O6) and the calculator adds up the IUPAC standard atomic weights, or type the molar mass from the reagent bottle. For hydrates, use the hydrated formula if that is what you weigh.
Worked examples
- 0.1 M NaCl, 500 mL: m = 0.1 × 0.5 × 58.44 = 2.922 g
- 5.844 g NaCl in 500 mL: c = 5.844 ÷ 58.44 ÷ 0.5 = 0.2 mol/L
- 1 M glucose (C6H12O6, 180.156 g/mol), 250 mL: m = 1 × 0.25 × 180.156 = 45.039 g
Frequently asked questions
What is the formula for molarity?
M = moles of solute ÷ litres of solution. With a mass, moles = grams ÷ molar mass.
What is the difference between molarity and molality?
Molarity is per litre of solution (mol/L); molality is per kilogram of solvent (mol/kg). Molality does not change with temperature.
What does 1 M mean?
One mole of solute in each litre of solution, e.g. 58.44 g of NaCl made up to 1 L.
How do I make 500 mL of 0.1 M sodium chloride?
Weigh out 2.92 g of NaCl, dissolve it in less than 500 mL of water, then top it up to exactly 500 mL.
How do I dilute a stock solution?
Use C₁V₁ = C₂V₂. For 100 mL of 0.1 M from a 1 M stock, take 10 mL of the stock and add water up to 100 mL.
Why top up to the volume instead of adding the solute to the full amount of water?
The solute takes up some room of its own. Dissolve it first, then fill a volumetric flask to the line, and the concentration comes out right.