1.1. A student investigated the reaction between aqueous sodium carbonate and two different solutions of dilute hydrochloric acid, A and B.

The reaction is:

Na₂CO₃ (aq) + 2HCl (aq) → 2NaCl (aq) + H₂O (l) + CO₂ (g)

Three experiments were carried out.

Experiment 1

Using a measuring cylinder, 25 cm³ of aqueous sodium carbonate were poured into a conical flask.

Thymolphthalein indicator was added to the conical flask. A burette was filled up to the 0.0 cm³ mark with solution A. Dilute hydrochloric acid A was added to the flask, until the solution just changed colour. Use the burette diagram to record the reading in the table.

Experiment 2

Experiment 1 was repeated using methyl orange indicator instead of thymolphthalein.

Methyl orange is red-orange in acidic solutions and yellow in alkaline solutions. Use the burette diagrams to record the readings in the table and complete the table. 

1.2. What colour change was observed in the flask in experiment 2?  from .......................................... to ........................................ 

1.3. Experiment 3

Experiment 1 was repeated using solution B of acid instead of solution A. Use the burette diagrams to record the readings in the table and complete the table.

1.4. Suggest one observation, other than colour change, that is made when hydrochloric acid is added to sodium carbonate.

1.5. Complete the sentence below.

Experiment .......... needed the largest volume of hydrochloric acid to change the colour of the indicator.

1.6. What would be the effect on the results, if any, if the solutions of sodium carbonate were warmed before adding the hydrochloric acid? Give a reason for your answer.

effect on results ..........................................................

reason ..........................................................

1.7. i) Determine the ratio of volumes of dilute hydrochloric acid used in experiments 1 and 3.

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ii) Use your answer to (h)(i) to deduce how the concentration of solution A differs from that of solution B.

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1.8. Suggest a different method, using standard laboratory chemicals, to determine which of the solutions of dilute hydrochloric acid, A or B, is more concentrated.