How to Calculate Molarity in Chemistry

c = n ÷ V

Insert your moles and volume, and the result gives the solution’s molarity.

Worked Example (IB-Style)

Question:
What is the molarity of a solution prepared by dissolving 4.00 g of NaOH in water to make 250 cm³ of solution?

Step 1: Calculate moles
Mr of NaOH = 40.0
n = 4.00 g ÷ 40.0 g/mol = 0.100 mol

Step 2: Convert volume
250 cm³ = 0.250 dm³

Step 3: Calculate molarity
c = 0.100 ÷ 0.250 = 0.400 mol dm⁻³

This type of problem appears frequently in Papers 1, 2, and 3.

Rearranging the Molarity Formula

IB questions often require solving for different variables.

Find moles:

n = c × V

Find volume:

V = n ÷ c

These rearrangements are essential when working with titration calculations.

Molarity in Dilution Problems

Dilutions follow the formula:

c₁V₁ = c₂V₂

Where:

• c₁ = initial concentration
• V₁ = initial volume
• c₂ = final concentration
• V₂ = final volume

This relationship allows you to dilute stock solutions accurately.

Example:
5.00 cm³ of 2.0 M solution diluted to 100 cm³
c₂ = (2.0 × 5.00) ÷ 100 = 0.10 M

Molarity in Stoichiometry and Titrations

Because molarity is tied to moles, it is a key part of:

• Determining limiting reagents in solution
• Calculating titration endpoints
• Acid–base neutralization calculations
• Redox titrations
• Equilibrium constant (Kc) calculations
• Reaction rate expressions involving solutions

IB assessment frequently combines molarity with balanced equations.

Common Mistakes to Avoid

• Forgetting to convert cm³ → dm³
• Dividing by mass instead of moles
• Using moles of wrong reactant (limiting reagent errors)
• Confusing molarity (mol dm⁻³) with molality (mol kg⁻¹)
• Rounding too early—IB prefers proper significant figures

Accurate unit handling is the biggest factor in correct answers.

FAQs

Is molarity temperature dependent?

Yes. Because volume changes with temperature, molarity can vary. This is why standard solutions are prepared at controlled conditions.

What if the solute dissociates into ions?

Molarity refers to the concentration of the compound, not the ions. Ion concentrations are calculated only after dissociation, using stoichiometry.

Can solids have molarity?

No. Molarity applies only to solutions. Solid concentrations use different units such as mol per gram or mol per kilogram.

Conclusion

Molarity is a simple yet powerful way to express solution concentration. By converting volumes correctly, calculating moles accurately, and applying c = n/V, you can handle titration problems, dilution questions, and stoichiometric calculations with confidence. Mastering molarity is essential for success in IB Chemistry and for understanding how solutions behave in real laboratory scenarios.

Learn why the mole allows chemists to connect atomic-scale particles to measurable quantities used in real experiments.