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Introduction to titration

A titration finds an unknown concentration by adding a known one drop by drop until the reaction is exactly complete. The subtle part is that 'complete' means matching moles by the reaction's ratio, not matching volumes or waiting for the indicator alone.

§1

Reaching the equivalence point.

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A titration measures an unknown concentration by adding a titrant of known concentration until the reaction is just complete. Typically an acid is titrated with a base, or vice versa.

The equivalence point is reached when the moles of titrant added exactly match the moles of analyte according to the reaction's stoichiometric ratio — not when equal volumes are mixed.

In the lab, an indicator signals the endpoint with a color change, used to estimate the equivalence point. The endpoint and the true equivalence point are close but not identical.

UNIT 4 TOPIC 4.6 • INTRODUCTION TO TITRATION TITRATION CURVE BUILDER LAB SETUP Titrant: known concentration NaOH(aq) in buret acid analyte + indicator EQUIVALENCE POINT pH Volume of titrant added (mL) equivalence point At equivalence: moles acid reacted = moles base added For a 1:1 acid:base, n(H⁺) = n(OH⁻) the steep midpoint of the jump ENDPOINT Observable signal colorless pink indicator color changes near the equivalence point Endpoint = the observed color change. Equivalence = the exact mole relationship. A good indicator makes them nearly coincide. WHAT TITRATION MEASURES Known titrant volume × concentration reveals the unknown amount of analyte — the reaction must be specific and quantitative. AP Chemistry · Unit 4 · Chemical Reactions
Fig. 4.6.1 In a titration, a titrant of known concentration is added to an analyte until the equivalence point — where the moles match the balanced equation's ratio. The endpoint (indicator color change) estimates that point; the two are not identical.
§2

Working a titration.

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Reason in moles, using the reaction's ratio.

  1. Find moles of titrant added. Multiply the titrant's concentration by the volume added: moles = M × V.
  2. Apply the stoichiometric ratio. Use the balanced equation to convert moles of titrant to moles of analyte — it is not always 1:1.
  3. Find the analyte concentration. Divide moles of analyte by the analyte's volume.
  4. Distinguish endpoint from equivalence. The indicator's endpoint estimates the equivalence point; choose an indicator whose color change is near it.
§3

The pieces you'll meet.

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A few terms, all pointing at moles.

titrant
Titrant
The solution of known concentration being added.
analyte
Analyte
The solution of unknown concentration being measured.
equivalence
Equivalence point
Moles of titrant and analyte match the stoichiometric ratio.
endpoint
Endpoint
The indicator's color change, used to estimate equivalence.
ratio
Stoichiometric ratio
The balanced coefficients; not always 1:1.
M×V
Moles
Concentration times volume gives moles.
§4

Worked example: a 1:2 titration.

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Setup. Titrate an unknown Ca(OH)₂ solution with HCl: Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O. The ratio is 1 Ca(OH)₂ : 2 HCl.

Moles of titrant. Suppose it takes 0.040 mol of HCl to reach equivalence.

Apply the ratio. Moles of Ca(OH)₂ = 0.040 mol HCl × (1 Ca(OH)₂ / 2 HCl) = 0.020 mol. Assuming 1:1 here would double the answer — the ratio matters.

Concentration. Divide 0.020 mol by the analyte's volume to get its molarity. Equal volumes of the two solutions would not have signaled equivalence; the moles and the 1:2 ratio do.

§5

Mistakes that cost real points.

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Pitfall · 01

"Equivalence is reached when equal volumes of the two solutions are mixed."

Equivalence depends on matching moles by the stoichiometric ratio, not on equal volumes. Solutions of different concentrations, or reactions with a non-1:1 ratio, reach equivalence at unequal volumes.

Fix. Find equivalence by moles (M × V) and the balanced ratio, not by comparing volumes.

Pitfall · 02

"The indicator endpoint is exactly the equivalence point."

The endpoint (color change) is an estimate of the equivalence point, close but not identical. A well-chosen indicator changes color very near equivalence, but they are distinct concepts.

Fix. Treat the endpoint as an estimate of equivalence; pick an indicator whose color change lands near the equivalence point.

Pitfall · 03

"Assume a 1:1 ratio between titrant and analyte."

The ratio comes from the balanced equation and is often not 1:1. For Ca(OH)₂ + 2HCl, it is 1:2. Assuming 1:1 gives the wrong concentration by a factor equal to the true ratio.

Fix. Read the stoichiometric ratio from the balanced equation, and use it to convert titrant moles to analyte moles.

§6

Skill Check.

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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.

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