How Does Solution Dilution Work?
Dilution is adding solvent (usually water) to a solution to lower its concentration. The key insight is that the number of moles stays the same — only the volume increases.
When diluting, moles before = moles after: M₁V₁ = M₂V₂. The molarity decreases as volume increases.
- 1↓StartConcentrated solution M₁, volume V₁
- 2↓Add solventPour pure solvent into flask
- 3↓MixStir to uniform concentration
- 4EndDilute solution M₂, volume V₂ (M₂ < M₁)
Try it: interactive calculator
Step-by-step worked examples
You have 100 mL of 2 M HCl. Dilute to 500 mL. Final molarity?
M₁V₁ = M₂V₂ 2 × 0.1 = M₂ × 0.5 M₂ = 0.2/0.5 = 0.4 M
Start: 250 mL of 0.8 M NaOH. Add water to 1 L total. Molarity?
M₁V₁ = M₂V₂ 0.8 × 0.25 = M₂ × 1 M₂ = 0.2 M
To make 2 L of 0.05 M from 0.5 M stock, what volume needed?
M₁V₁ = M₂V₂ 0.5 × V₁ = 0.05 × 2 V₁ = 0.1/0.5 = 0.2 L = 200 mL
Flashcards
Quick quiz
Q1.0.5 M, 200 mL diluted to 1 L. Final M?
Q2.Dilution = adding what?
Q3.Moles of solute during dilution…
Q4.3 M, 100 mL → 300 mL. Final M?
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Common mistakes
Adding molarities: 2 M + 1 M = 3 M after mixing. — Correct: Use M₁V₁ = M₂V₂. Molarity doesn't add linearly.
Forgetting to convert mL to L. — Correct: Always work in litres (100 mL = 0.1 L).
Thinking solute increases in dilution. — Correct: Solute is constant; only volume increases.
Mixing volumes: 100 + 200 mL = combined M? — Correct: Use M₁V₁ + M₂V₂ only if both started as separate solutions; dilution uses M₁V₁ = M₂V₂.
FAQ
What is dilution formula?
M₁V₁ = M₂V₂ — initial molarity × volume = final molarity × volume.
Why doesn't molarity just divide?
Because volume changes in the denominator. M₁V₁ = M₂V₂ accounts for both.
Can you reverse a dilution?
No — evaporating water is separate. Dilution adds solvent, reversing requires removing it.
Dilution in lab practice?
Pour stock into flask, add solvent to mark, mix. Never dilute acid by water — water to acid instead.