QUANTITATIVE ANALYSIS — PART 1

WAEC Chemistry Practical titration masterclass. This lesson introduces students to quantitative analysis, acid-base titration, burette reading, concordant titres, mole ratio, concentration calculations and examiner-style reporting.

Part 1 focuses on the common WAEC titration pattern: acid in the burette, alkali in the conical flask, indicator endpoint, titration table and calculations.

1. What Quantitative Analysis Means

Quantitative analysis is the branch of chemistry practical that determines the amount, concentration, mass or percentage composition of a substance. In WAEC Chemistry Practical, the most common quantitative analysis is titration.

Titration is a laboratory method used to find the concentration of a solution by reacting it with another solution of known concentration.

Titration Setup

Burette, pipette, conical flask, indicator and white tile are arranged to measure reacting volumes accurately.

Burette Reading

The burette reading must be read from the bottom of the meniscus at eye level and recorded to two decimal places.

Concordant Titres

Two or more close titre values are used to calculate the average titre. The rough titre is not averaged.

WAEC MARKING WARNING: Do not average the rough titre. Use only concordant accurate titres.

2. Apparatus and Their Functions

ApparatusFunctionWAEC Accuracy Point
BuretteDelivers acid or alkali gradually during titration.Read to 2 decimal places, e.g. 12.10 cm³.
PipetteMeasures a fixed volume, usually 20.0 cm³ or 25.0 cm³.Rinse with the solution it will measure.
Conical flaskHolds the solution being titrated.Can be rinsed with distilled water only.
White tileMakes endpoint colour change clear.Prevents overshooting the endpoint.
IndicatorShows the endpoint by colour change.Use 2–3 drops only.
Retort stand and clampHolds the burette vertically.Burette must not be slanted.
Labelled titration setup Teacher guide: Label the burette, clamp, retort stand, acid A, conical flask, alkali B, white tile and indicator. The burette must stand vertically so the reading is accurate.

3. Core Theory of Acid-Base Titration

In this WAEC model, A is dilute tetraoxosulphate(VI) acid, \(H_2SO_4\), and B is sodium hydroxide solution, \(NaOH\).

\[ H_2SO_4(aq)+2NaOH(aq)\rightarrow Na_2SO_4(aq)+2H_2O(l) \]

The mole ratio from the balanced equation is:

\[ H_2SO_4 : NaOH = 1 : 2 \]
This is the most important part of the calculation. If the mole ratio is wrong, the final concentration will be wrong.

Useful Formulae

\[ n = CV \]

where \(n\) = number of moles, \(C\) = concentration in \(mol\,dm^{-3}\), and \(V\) = volume in \(dm^3\).

\[ \frac{C_A V_A}{C_B V_B}=\frac{n_A}{n_B} \]

For \(H_2SO_4\) and \(NaOH\):

\[ \frac{C_A V_A}{C_B V_B}=\frac{1}{2} \]

4. WAEC Titration Procedure

Aim

To determine the concentration of dilute \(H_2SO_4\) using a standard NaOH solution.

Procedure

  1. Rinse the burette with A and fill it with A.
  2. Remove air bubbles from the jet of the burette.
  3. Record the initial burette reading.
  4. Rinse the pipette with B.
  5. Pipette 25.0 cm³ of B into a conical flask.
  6. Add 2–3 drops of phenolphthalein.
  7. Place the conical flask on a white tile.
  8. Run A from the burette into B while swirling continuously.
  9. Near endpoint, add A drop by drop.
  10. Stop when the pink colour just disappears.
  11. Record the final burette reading.
  12. Repeat until concordant titres are obtained.
Correct endpoint for phenolphthalein when NaOH is in the flask: pink to colourless.

Teacher

Students, titration is not pouring acid into alkali. It is controlled addition. Your hand controls the burette tap, your other hand swirls the flask, and your eyes watch the colour.

Student

Sir, why do we add the acid dropwise near the endpoint?

Teacher

Because one extra drop can overshoot the endpoint and make your titre too high. WAEC rewards careful endpoint control.

Practical Reality

  • If you stop too early, titre becomes too low.
  • If you overshoot, titre becomes too high.
  • If air bubble remains in the burette jet, titre becomes inaccurate.
  • If the burette is not vertical, reading becomes unreliable.

5. Correct WAEC Titration Table

A good titration table must contain initial reading, final reading and volume used. Readings must be recorded to two decimal places.

TitrationRough1st2nd3rd
Final burette reading / cm³12.4012.1512.1012.15
Initial burette reading / cm³0.000.000.000.00
Volume of A used / cm³12.4012.1512.1012.15
\[ Average\ titre=\frac{12.15+12.10+12.15}{3}=12.13cm^3 \]
WAEC TIP: The rough titre is used only to locate the endpoint. It is not part of the average.

Teacher

The table itself carries marks. Even if your calculations are correct, a poor table can reduce your score.

How WAEC Sees the Table

  • Correct headings attract marks.
  • Correct units attract marks.
  • Readings to two decimal places attract marks.
  • Concordant titres attract marks.
  • Correct average attracts marks.

6. Full WAEC Calculation Class

Given

  • B contains \(4.0g\,dm^{-3}\) of NaOH.
  • Volume of B pipetted = \(25.0cm^3\).
  • Average volume of A used = \(12.13cm^3\).
  • Equation: \(H_2SO_4 + 2NaOH \rightarrow Na_2SO_4 + 2H_2O\).

Step 1: Concentration of B

\[ Molar\ mass\ of\ NaOH=23+16+1=40g\,mol^{-1} \] \[ C_B=\frac{4.0}{40}=0.100mol\,dm^{-3} \]

Step 2: Concentration of A

\[ \frac{C_A V_A}{C_B V_B}=\frac{1}{2} \] \[ \frac{C_A\times12.13}{0.100\times25.0}=\frac{1}{2} \] \[ C_A=\frac{0.100\times25.0}{2\times12.13}=0.103mol\,dm^{-3} \]

Step 3: Mass Concentration of A

\[ Molar\ mass\ of\ H_2SO_4=2(1)+32+4(16)=98g\,mol^{-1} \] \[ Mass\ concentration=0.103\times98=10.1g\,dm^{-3} \]

Teacher

Students, never rush titration calculations. Start from the equation. The equation tells you the mole ratio.

Student

Sir, why did we divide by 2?

Teacher

Because 1 mole of \(H_2SO_4\) reacts with 2 moles of NaOH. Therefore the acid mole value is half the alkali mole value for the reacting amounts.

Industry Connection

Titration is used in water treatment, food industries, pharmaceutical quality control, fertilizer production, soap making and environmental monitoring.

7. Errors That Affect Titre Values

ErrorEffectReason
Overshooting endpointTitre too highToo much acid added.
Stopping before endpointTitre too lowNeutralization incomplete.
Air bubble in burette jetTitre unreliable/usually highPart of burette volume fills air space first.
Burette rinsed with water onlySolution in burette dilutedConcentration changes.
Pipette not rinsed with BB may be dilutedMeasured moles of B reduce.
Parallax errorWrong readingEye not level with meniscus.
FINAL WARNING: In WAEC, values alone are not enough. You must show working, units and correct mole ratio.

8. Part 1 Exam-Ready Summary

Quantitative Analysis

Determines amount or concentration of a substance.

Titration

A method for finding unknown concentration using a standard solution.

Phenolphthalein Endpoint

Pink to colourless when acid is added to alkali.

Mole Ratio

\(H_2SO_4:NaOH = 1:2\).

Concordant Titres

Close titre values used for averaging. Rough titre is ignored.

Accuracy Rule

Read burette at eye level and record to 2 decimal places.

You are ready for Quantitative Analysis Part 2: advanced titration calculations, dilution, percentage purity and WAEC problem-solving.