Acid–Base Titration Virtual Lab

Aim

(a) To find the molarity and strength of a given hydrochloric acid solution by titrating it against standard 0.100 M sodium hydroxide. (b) To find the molarity of a sodium hydroxide solution by titrating it against standard 0.0500 M oxalic acid, as in the CBSE Class 11 practical.

Apparatus and materials

• 50 mL burette with stand and clamp, 10 mL pipette, 100 mL conical flask, white glazed tile, funnel
• 0.100 M NaOH (for part a), HCl of unknown concentration
• 0.0500 M oxalic acid (6.30 g of H₂C₂O₄·2H₂O per litre) and NaOH of unknown concentration (for part b)
• Phenolphthalein indicator (methyl orange for comparison)

Theory

In a neutralisation titration a solution of known molarity is added from a burette to a measured volume of the other solution until the moles of H⁺ and OH⁻ are exactly equal. That moment is the equivalence point; the end point is where the indicator changes colour, and a good indicator makes the two coincide.

The pH curve

The simulator calculates the flask's pH after every drop by solving the charge balance, including water's own ionisation (K_w = 1.0 × 10⁻¹⁴) and, for oxalic acid, both dissociation steps (K_a1 = 5.6 × 10⁻², K_a2 = 5.4 × 10⁻⁵). For HCl the pH stays low while acid is in excess, then rises vertically from about 4 to 10 within a fraction of a millilitre around equivalence (pH 7), and levels off near 12 once NaOH is in excess. For oxalic acid the curve starts higher, rises more gradually and the equivalence point is basic, because the oxalate ion is a weak base. Phenolphthalein (pH 8.2–10) sits on the vertical part of both curves; methyl orange (pH 3.1–4.4) suits only the strong acid.

Reading a titration curve, choosing an indicator and calculating pH after partial neutralisation are standard JEE Main and NEET ionic-equilibrium questions, so it is worth watching how the curve changes when you switch the experiment.

Procedure

1. Rinse the burette with NaOH, fill it, remove the air bubble from the nozzle and note the initial reading at the bottom of the meniscus.
2. Rinse the pipette with the acid and transfer 10.00 mL into a clean conical flask. Add 1–2 drops of phenolphthalein.
3. Place the flask on a white tile under the burette. Run in NaOH while swirling. This first, rough trial tells you roughly where the end point is.
4. Repeat. This time add NaOH quickly to about 1 mL before the rough end point, then drop by drop, swirling after each drop.
5. Stop when one drop gives a faint pink colour that does not fade on swirling. Note the final reading.
6. Repeat until two readings are concordant (within 0.1 mL).

Observations

Volume of acid in the flask, V₁ = 10.00 mL. Molarity of NaOH, M₂ = 0.100 M (part a).

Sample working: trials 2 and 3 agree within 0.1 mL, so the concordant volume is V₂ = (12.15 + 12.10) / 2 = 12.13 mL. M₁ = M₂V₂ / V₁ = 0.100 × 12.13 / 10.00 = 0.1213 M. Strength = 0.1213 × 36.5 = 4.43 g L⁻¹.

Precautions

• Remove the air bubble from the burette nozzle before taking the initial reading.
• Read the lower meniscus with your eye level with it (the upper meniscus for coloured solutions such as KMnO₄).
• Never blow out the last drop from the pipette.
• Add the titrant drop by drop near the end point and swirl continuously.
• Use only 1–2 drops of indicator; indicators are weak acids themselves.
• A deep pink colour means you have overshot. Repeat that trial.

Viva questions with answers

1. What is a standard solution?

A solution whose concentration is known accurately. Oxalic acid is a primary standard because it is pure, stable and not hygroscopic; NaOH is not, since it absorbs moisture and CO₂.

2. Why is phenolphthalein colourless in acid and pink in base?

It is a weak acid (HIn). The neutral form is colourless; above pH about 8.2 it loses protons to form the pink, conjugated In²⁻ ion.

3. What is the pH at the equivalence point of HCl vs NaOH?

7 at 25 °C, because NaCl does not hydrolyse.

4. Why is the equivalence point of oxalic acid vs NaOH above 7?

Sodium oxalate is the salt of a weak acid and strong base; the oxalate ion hydrolyses and makes the solution slightly basic.

5. What is the basicity of oxalic acid?

Two, because each molecule has two replaceable H⁺ ions.

6. Why do we not rinse the conical flask with the acid?

Drops left behind would add an unknown extra amount of acid to the pipetted volume and give a higher titre.

Related practice

Try the flame test lab and the salt analysis lab, or practise numerical problems on titration, molarity and pH with previous-year JEE Main questions and the question bank.

Frequently asked questions