Molarity and Preparing Solutions: From Molar Mass to Grams to Weigh

Calculate how many grams to weigh for a solution of a given molarity, dilute stock solutions with C1V1 = C2V2, and avoid common preparation mistakes.

By the Dayob Editorial Team · Updated · 2 min read

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Molarity (M) is the most common way to express concentration in chemistry: moles of solute per liter of solution. Preparing a solution of a given molarity takes three numbers: the concentration you want, the volume you need, and the molar mass of the solute.

The key formula

grams = molarity (mol/L) × volume (L) × molar mass (g/mol)

Example: 500 mL of 0.25 M sodium chloride

  • Moles needed: 0.25 mol/L × 0.500 L = 0.125 mol
  • Molar mass of NaCl: 58.44 g/mol
  • Mass to weigh: 0.125 × 58.44 = 7.31 g

Find the molar mass of any solute with the molecular weight calculator; the "Amount (mol)" box then converts moles to grams directly.

How to prepare it

  1. Weigh the solute accurately.
  2. Dissolve it in somewhat less than the final volume of distilled water.
  3. Transfer to a volumetric flask, rinsing the beaker into the flask.
  4. Add water up to the calibration mark, then cap and invert to mix.

Molarity is per liter of solution, not per liter of water added, which is why you fill to the mark rather than measuring water first.

Dilutions: C₁V₁ = C₂V₂

When diluting a stock solution, the moles of solute stay the same, so concentration × volume is constant.

Example: make 250 mL of 0.10 M HCl from 1.0 M stock.

1.0 × V₁ = 0.10 × 250  →  V₁ = 25 mL

Measure 25 mL of stock and dilute to a total of 250 mL. This is a proportion problem at heart; see how to solve proportions.

Safety: When diluting concentrated acids, always add acid slowly to water, never water to acid, and wear appropriate eye and hand protection.

Molarity vs. other concentration units

UnitDefinitionWhen it's used
Molarity (M)mol solute / L solutionMost lab work
Molality (m)mol solute / kg solventFreezing point and boiling point problems; doesn't change with temperature
Mass percentg solute / 100 g solutionCommercial products, food
ppmmg solute per kg (≈ per L for dilute water solutions)Water quality, trace amounts

Common mistakes

  • Using milliliters without converting to liters (500 mL = 0.500 L).
  • Ignoring water of hydration in the solute; see hydrates.
  • Adding the full volume of water to the solid instead of filling to the mark.
  • Using the wrong formula for the solute, such as Ca(OH)2 vs. CaO.

Further reading from official sources

More molecular weight guides

A round glass dish on white paper in a laboratoryMolar Mass vs. Molecular Weight vs. Formula Mass: What's the Difference?Clear definitions of molar mass, molecular weight, formula mass and atomic mass units, why their numbers match, and how to calculate each from a chemical formula.3 min readA scientist using a pipette with test tubes in a labGrams to Moles and Moles to Grams: Conversions with Worked ExamplesConvert between grams, moles and number of particles using molar mass and Avogadro's number, with step-by-step examples and a stoichiometry walkthrough.2 min readTest tubes filled with different colored liquidsPercent Composition by Mass: How to Calculate It and Why It MattersCalculate the mass percent of each element in a compound from its formula, check the result, and use percent composition in labs, nutrition labels and fertilizers.2 min read