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Molarity Calculator

Find the grams of a substance to weigh out for a solution of a given molarity, or the molarity of a solution from the mass and volume.

Rules last checked 10 November 2026Every step shown

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mol/L
mL

Mass to weigh out

7.305 g

In milligrams
7,305 mg
Amount of substance
0.125 mol

Weigh out 7.305 g of sodium chloride, dissolve it in less water than you need, then make it up to 250 mL for a 0.5 M solution.

How it's worked out

  1. Moles needed0.5 mol/L × 0.25 L0.125 mol
  2. Mass0.125 mol × 58.44 g/mol7.305 g
  • Hydrated salts weigh more per mole than the dry form: use the molar mass of exactly what is in the bottle.

How it is worked out

Moles needed = molarity × volume in litres. Mass = moles × molar mass. Going the other way, moles = mass ÷ molar mass, and molarity = moles ÷ litres.

Diluting afterwards

To make a weaker solution from a stronger one you've already made, use the dilution calculator, which works out the stock and water to mix.

Questions people ask

How many grams of salt make 250 mL of a 0.5 M solution?

7.305 g of sodium chloride (NaCl, 58.44 g/mol): 0.5 mol/L × 0.25 L = 0.125 mol, and 0.125 × 58.44 = 7.305 g.

What is molarity?

The number of moles of a substance in one litre of solution, written mol/L or M. A 1 M solution of NaCl has 58.44 g of salt in each litre.

Where do I find the molar mass?

On the bottle or the supplier's data sheet, or by adding up the atomic masses in the formula. Use the form you actually have: hydrated salts such as copper sulfate pentahydrate weigh more per mole than the dry form.

Is it per litre of water or per litre of solution?

Per litre of finished solution. Dissolve the substance in less water than you need, then make it up to the final volume.

Formula and rules last checked 10 November 2026Written and tested by the CalculatorDex teamReport a problem