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Molarity and Dilution Explained

By the Periodixy Editorial Team · Last reviewed July 10, 2026

Molarity is chemistry's standard way to describe how concentrated a solution is: moles of dissolved substance per litre of solution. Dilution is the everyday lab operation of adding solvent to reduce that concentration. One formula governs each — and both are shorter than most recipes.

A volumetric flask used to prepare solutions of known concentration
Photo by Jeff Burkholder on Pexels

Molarity: moles per litre

Definition

M = moles of solute ÷ litres of solution

Making a 1 M salt solution

How would you prepare 1 L of 1 M NaCl?

  1. Molar mass of NaCl ≈ 58.44 g/mol — so 1 mole is 58.44 g.
  2. Dissolve 58.44 g of NaCl in water.
  3. Top up with water to exactly 1 L of total solution.

Answer: 58.44 g of NaCl made up to 1 L gives a 1.00 M solution.

⚠ Common mistake: Molarity uses litres of final solution, not litres of water added. Dissolving salt in 1 L of water gives slightly more than 1 L of solution — real labs use volumetric flasks to hit the final volume exactly.

The dilution equation

Dilution

C₁V₁ = C₂V₂

The logic: diluting adds water but not solute, so the moles of solute stay constant. Since moles = concentration × volume, the product C × V must be the same before (1) and after (2).

Classic lab problem

How much 2.0 M stock solution do you need to make 100 mL of 0.5 M solution?

  1. Solve for V₁: V₁ = C₂V₂ ÷ C₁
  2. V₁ = (0.5 M × 100 mL) ÷ 2.0 M

Answer: 25 mL of stock, then add water up to 100 mL. Verify with the Dilution Calculator.

💡 Tip: You can keep volumes in mL on both sides — the units cancel. Just never mix mL on one side with L on the other. Unit consistency causes more wrong answers than the algebra does.

Molarity from a measured mass

Real problems often start from grams. Chain two steps: grams → moles (divide by molar mass), then moles → molarity (divide by litres).

Grams to molarity

10.0 g of glucose (C₆H₁₂O₆, 180.16 g/mol) is dissolved to make 250 mL of solution. Find the molarity.

  1. moles = 10.0 ÷ 180.16 = 0.0555 mol
  2. volume = 0.250 L
  3. M = 0.0555 ÷ 0.250

Answer: ≈ 0.222 M

Summary

  • Molarity M = moles ÷ litres of final solution.
  • Dilution: C₁V₁ = C₂V₂ — moles of solute don't change when you add water.
  • From mass: grams → moles (÷ molar mass) → molarity (÷ litres).
  • Keep units consistent on both sides of every equation.

Frequently asked questions

What does a capital M mean in chemistry?

Molar — mol/L. A “0.5 M solution” contains half a mole of solute in every litre of solution.

Is molarity the same as molality?

No. Molarity (M) is moles per litre of solution; molality (m) is moles per kilogram of solvent. Molality is used when temperature changes matter, since volume expands but mass doesn't.

Can C₁V₁ = C₂V₂ be used for mixing two solutions?

No — it only describes diluting one solution with pure solvent. Mixing two solutions of the same solute needs a moles-total calculation.

Practise with these tools

Molarity CalculatorCalculate molarity, moles, volume, or concentration from mass — with steps shown.Dilution Calculator (C₁V₁ = C₂V₂)Solve dilution problems with automatic unit conversion between M, mM, µM and L, mL, µL.Molar Mass CalculatorCalculate the molar mass of any chemical formula with a full element-by-element breakdown and percent composition.

Related study guides

Molar Mass: Definition and How to Calculate ItWhat is molar mass, and how do you find it? Add up the atomic masses in a formula to get grams per mole. Worked examples for H2O, glucose, Ca(OH)2 and NaCl.pH and pOH ExplainedUnderstand the pH and pOH scales: the formulas pH = −log[H⁺] and pH + pOH = 14, worked examples, and why each pH step means 10× the acidity.Stoichiometry Step-by-StepA clear method for stoichiometry: balance the equation, convert to moles, apply the mole ratio, convert back. Includes full worked examples.