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Concentration and titration

How much acid is in this vinegar? How much chlorine is in the town's water? Is this medicine the strength the label says? Chemists answer questions like these by measuring concentrations precisely, and titration is one of their most accurate tools.

Recommended for MYP 5 · eAssessment priority · About 3 lessons · Criteria A, B, C and D

burettesodium hydroxide, 0.100 mol/dm³scale read to 0.05 cm³tapconical flask25.0 cm³ hydrochloric acid(measured by pipette) + indicatorwhite tile(shows the end point)clamp stand
Figure 1. In a titration, one solution is added from a burette until the indicator just changes colour.
On this page
  1. Learning objectives
  2. Before you start
  3. Key vocabulary
  4. Understanding the ideas
  5. Titration calculations in four steps
  6. Concentration in the real world
  7. Worked examples
  8. In the eAssessment
  9. Check your understanding
  10. Practice questions
  11. Investigation
  12. Criterion-linked questions
  13. Challenge questions
  14. Topic check
  15. Review your mistakes
  16. Your progress

Learning objectives

By the end of this topic you should be able to:

  • calculate concentrations in g/dm³ and mol/dm³
  • calculate the effect of diluting a solution
  • describe how to carry out an accurate acid–alkali titration
  • calculate unknown concentrations from titration results, using balanced equations
  • process titration results and evaluate their reliability
  • discuss the importance of concentration in health and water treatment

Before you start

You will use these skills. If any feel shaky, review them first.

  • the mole and reacting masses (see The mole and reacting masses)
  • acids, alkalis and neutralisation
  • balancing equations

Key vocabulary

Concentration
The amount of solute per unit volume: g/dm³ or mol/dm³.
Standard solution
A solution whose concentration is known accurately.
Titre
The volume of solution added from the burette to reach the end point.
End point
The point at which the indicator changes colour, showing neutralisation is complete.
Concordant results
Titres that agree within 0.10 cm³.
Dilution
Adding solvent, which lowers concentration while the amount of solute stays the same.

Understanding the ideas

  1. What is it?

    Concentration tells you how much solute is dissolved in a given volume. In chemistry it is usually given in mol/dm³: concentration = moles ÷ volume (dm³). To convert g/dm³ into mol/dm³, divide by the relative formula mass.

  2. Why does it happen?

    In an acid–alkali titration, a measured volume of one solution is placed in a conical flask with an indicator, and the other solution is added from a burette until the indicator just changes colour. If one concentration is known, the balanced equation lets you calculate the other.

  3. How do we know?

    Accurate titration needs careful technique: rinsing apparatus with the right solutions, reading the burette at eye level, adding dropwise near the end point and repeating until results are concordant. Laboratories in hospitals, food factories and water works use titration and similar methods every day.

  4. Why does it matter?

    Concentrations matter in medicine doses, drinking-water treatment, food labelling and pollution monitoring. Getting them wrong can make a treatment useless or harmful.

  5. What does it connect to?

    This topic links to the mole, acids and neutralisation in chemistry, to medicine and digestion in biology, and to ratios and percentages in mathematics.

Titration calculations in four steps

  1. Moles of the known solution: n = c × V (with V in dm³: divide cm³ by 1000).
  2. Use the equation ratio: for example 1 : 1 for HCl + NaOH, 1 : 2 for H₂SO₄ + 2NaOH.
  3. Concentration of the unknown: c = n ÷ V (in dm³).
  4. Convert if needed: g/dm³ = mol/dm³ × Mr; multiply by any dilution factor.

Concentration in the real world

Water works measure chlorine levels to keep drinking water safe. Pharmacists check that medicine solutions have the right concentration. Food scientists titrate vinegar, fruit juices and milk to check their acidity.

Worked examples

Example 1: mol/dm³ from mass

5.3 g of sodium carbonate (Mr = 106) is dissolved to make 250 cm³ of solution. Find the concentration.

  1. Moles = 5.3 ÷ 106 = 0.050 mol.
  2. Volume = 0.250 dm³.
  3. Concentration = 0.050 ÷ 0.250 = 0.20 mol/dm³.

Example 2: a titration

20.0 cm³ of 0.150 mol/dm³ NaOH neutralises 25.0 cm³ of HCl. Find the HCl concentration.

  1. Moles NaOH = 0.150 × 0.0200 = 0.00300 mol.
  2. Ratio 1 : 1, so moles HCl = 0.00300 mol.
  3. Concentration = 0.00300 ÷ 0.0250 = 0.120 mol/dm³.

In the eAssessment

Titration questions reward careful, step-by-step working. Expect:

  • Calculate concentrations, moles and dilutions, converting cm³ to dm³.
  • Use balanced equations for acids that are not 1 : 1.
  • Process titration results: identify concordant titres and anomalies.
  • Describe and evaluate the titration method and its sources of error.

Common ways to lose marks: forgetting to divide cm³ by 1000; ignoring the ratio in the equation; including the rough titre in the mean; and confusing concentration with total amount.

Check your understanding

Quick questions on the ideas above. Try each one before using a hint.

Practice questions

Show

Investigation: comparing antacids by titration

Partially guided investigation · about 60 minutes · pairs

Research question
Which of three brands of antacid tablet neutralises the most hydrochloric acid per tablet, and which gives the best value per rupee?
Scientific background
Antacids contain bases such as calcium carbonate or magnesium hydroxide that neutralise excess stomach acid. Titrating a dissolved tablet with acid of known concentration measures how much acid it can neutralise.
Hypothesis
Write your own prediction, with a scientific justification.
Variables
Identify your independent, dependent and control variables, and explain how you will control them.
Apparatus
Three brands of antacid tablets, mortar and pestle, conical flasks, 0.10 mol/dm³ hydrochloric acid, burette, clamp stand, methyl orange indicator, distilled water, white tile.
Method
  1. Crush one tablet and mix it with 50 cm³ of distilled water in a conical flask; add 3 drops of methyl orange.
  2. Fill the burette with 0.10 mol/dm³ hydrochloric acid and record the start reading at eye level.
  3. Add acid while swirling, dropwise near the end, until the indicator turns permanently from yellow to orange-red.
  4. Repeat with a second tablet of each brand, and calculate the moles of acid neutralised per tablet and per rupee.

Safety. Wear eye protection. Dilute hydrochloric acid is an irritant; wipe up spills and wash skin with water. Do not taste the tablets or solutions in the laboratory.

Then evaluate: why is the end point harder to judge with a suspension than with a clear solution, and how could you make it sharper?

Criterion-linked questions

Criterion B: inquiring and designing

Criterion C: processing and evaluating

Criterion D: reflecting on the impacts of science

Challenge questions

Harder problems in unfamiliar contexts. Plan before you calculate.

Topic check

Five questions picked at random from the whole topic. Take a new set whenever you like.

Review your mistakes

Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.

Your progress

Tracked separately for each skill, on this device only.

SkillCorrectStatus

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