Weak Acid vs Strong Base Titration
Acetic Acid (CH₃COOH) vs Sodium Hydroxide (NaOH)
Complete Theory • Apparatus • Procedure • Observation • Calculation • pH Curve
Introduction
Hello, students! Today, we are going to understand a very important chemistry laboratory experiment: Weak Acid vs Strong Base Titration.
Before picking up the burette and pipette, we must understand what a weak acid and a strong base actually are. Let us begin with their definitions in simple language.
1. Definitions
Weak Acid
A weak acid is an acid that ionizes only partially in water, producing a relatively low concentration of hydrogen ions (H⁺).
Example: Acetic acid (CH₃COOH).
Strong Base
A strong base dissociates almost completely into ions in aqueous solution, producing hydroxide ions (OH⁻).
Example: Sodium hydroxide (NaOH).
2. Aim of the Experiment
Aim: To determine the molarity and strength of a given weak acid (CH₃COOH) solution by titrating it against a standard solution of a strong base (NaOH).
In simple words, we have to determine the concentration of the given acid using a sodium hydroxide solution of known concentration.
3. Apparatus and Chemicals Required
Apparatus
- Burette
- Pipette
- Conical flask
- Burette stand and clamp
- Funnel
- Wash bottle
- White tile
Chemicals
- Weak acid: CH₃COOH
- Standard NaOH solution
- Phenolphthalein indicator
- Distilled water
4. Principle of the Experiment
When a weak acid reacts with a strong base, a neutralization reaction takes place. Acetic acid reacts with sodium hydroxide to form sodium acetate and water.
The net ionic equation is commonly represented as:
The reaction occurs in a 1:1 molar ratio. Therefore, at the equivalence point, the number of moles of NaOH added equals the initial number of moles of CH₃COOH, provided the acid is acetic acid and the reaction proceeds as shown.
5. Why Is Phenolphthalein Used?
Phenolphthalein is a suitable indicator for this titration because its colour changes within the pH range of approximately 8.2–10.0.
- In acidic solution: Colourless.
- In its transition range: Very pale pink to pink.
- At the endpoint: A faint pink colour persists briefly.
The endpoint is reached when a very pale pink colour persists for about 30 seconds while the flask is swirled.
6. Step-by-Step Experimental Procedure
Wash the burette with distilled water, then rinse it with a small quantity of the standard NaOH solution. Fill the burette with NaOH, remove any air bubbles from the tip, and record the initial reading. The reading need not be exactly 0.00 mL.
Rinse the pipette with the given acid solution. Using the pipette, transfer 10.00 mL or 20.00 mL of the weak acid into a clean conical flask. Use the volume specified in your experiment.
Add 1–2 drops of phenolphthalein indicator to the acid in the conical flask. The solution should remain colourless.
Add NaOH from the burette slowly while continuously swirling the conical flask. As the endpoint approaches, add the titrant drop by drop.
Stop adding NaOH when a faint pink colour persists for about 30 seconds. Record the final burette reading.
Repeat the titration until you obtain concordant readings. Concordant readings are readings that agree closely, according to the tolerance specified by your laboratory.
7. Observation Table
The following table shows the illustrative readings for a 10 mL acid sample. These readings should be replaced by your actual laboratory observations when performing the experiment.
| Trial | Volume of Acid (mL) | Initial Burette Reading (mL) | Final Burette Reading (mL) | NaOH Used (mL) |
|---|---|---|---|---|
| 1 | 10.0 | 0.0 | 9.6 | 9.6 |
| 2 | 10.0 | 0.0 | 9.5 | 9.5 |
| 3 | 10.0 | 0.0 | 9.5 | 9.5 |
The second and third readings agree, so 9.5 mL is taken as the titre value for the following illustrative calculation.
8. Calculations
Given Data
- Volume of CH₃COOH, V₁ = 10.0 mL
- Molarity of NaOH, M₂ = 0.100 mol/L
- Volume of NaOH used, V₂ = 9.5 mL
- Molar mass of CH₃COOH = 60.05 g/mol (approximately)
Formula for Molarity
Where:
- M₁ = Molarity of the weak acid.
- V₁ = Volume of the weak acid.
- M₂ = Molarity of the standard NaOH solution.
- V₂ = Volume of NaOH used.
Step 1: Rearrange the Formula
Step 2: Substitute the Values
Step 3: Calculate
First, multiply 0.100 by 9.5:
Now divide by 10.0:
Therefore, the molarity of the given acetic acid solution is 0.095 M, based on the assumed standard NaOH concentration and the illustrative titre reading.
9. Calculation of Strength in g/L
The strength of a solution in grams per litre is calculated by multiplying its molarity by its molar mass.
For acetic acid:
- Molarity = 0.095 mol/L
- Molar mass = 60.05 g/mol
Therefore, the calculated strength of the acetic acid solution is approximately 5.70 g/L.
10. Titration Curve: Weak Acid vs Strong Base
A titration curve is a graph showing how the pH of the solution changes as the volume of NaOH added increases. The curve below is a schematic illustration of a weak acid–strong base titration.
Schematic curve only. The exact shape and equivalence volume depend on the actual concentrations and experimental conditions.
- The initial pH is acidic because CH₃COOH is a weak acid.
- The pH increases gradually as NaOH is added.
- A buffer region appears before the equivalence point.
- The equivalence point occurs above pH 7.
- After equivalence, excess NaOH causes the pH to rise further.
11. Precautions
- Rinse the burette with NaOH solution before filling it.
- Rinse the pipette with the acid solution before use.
- Ensure that no air bubbles remain in the burette tip.
- Read the burette at eye level.
- Add NaOH dropwise near the endpoint.
- Swirl the conical flask continuously during titration.
- Use only 1–2 drops of phenolphthalein unless instructed otherwise.
- Record the initial and final readings carefully.
- Repeat until concordant readings are obtained.
- Wear appropriate laboratory safety equipment.
12. Important Viva Questions
Q1. What is the weak acid used in this experiment?
Acetic acid (CH₃COOH).
Q2. Which strong base is used?
Sodium hydroxide (NaOH).
Q3. Which indicator is used?
Phenolphthalein.
Q4. What is the colour change at the endpoint?
Colourless to a faint, persistent pink.
Q5. Is the equivalence-point pH equal to 7?
No. It is greater than 7 for a weak acid–strong base titration under typical aqueous conditions.
Q6. What is the chemical equation?
CH₃COOH + NaOH → CH₃COONa + H₂O.
Q7. What is the molar ratio of acetic acid to NaOH?
The ratio is 1:1.
Q8. What is a concordant reading?
Readings that agree closely within the laboratory's accepted tolerance.
Q9. Why is NaOH called a strong base?
Because it dissociates almost completely into Na⁺ and OH⁻ ions in aqueous solution.
Q10. What is the formula for strength in g/L?
Strength = Molarity × Molar mass.
13. Final Result
Molarity of CH₃COOH: 0.095 mol/L
Strength of CH₃COOH: Approximately 5.70 g/L
These are calculated results based on the illustrative titre of 9.5 mL and an assumed 0.100 M NaOH solution.
Conclusion
In this experiment, a weak acid (acetic acid) is titrated against a standard strong base (sodium hydroxide). Phenolphthalein helps identify the endpoint through a persistent pale-pink colour. The acid concentration is calculated from the titre value, and its strength is obtained by multiplying molarity by molar mass.
Understanding the reaction, endpoint, observation table, calculation, and titration curve is essential for chemistry practical examinations and laboratory work.
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