4.2 - Standardisation of a Hydrochloric Acid Solution

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Experiment Summary

In this experiment, a solution of sodium carbonate ( $Na₂CO₃$ ) is used to standardise a hydrochloric acid ( $HCl$ ) solution of unknown concentration.

Sodium carbonate is a primary standard, meaning it can be used to accurately determine the concentration of $HCl$ through a titration process.

The neutralisation reaction between sodium carbonate and hydrochloric acid is:

\text{Na}_2\text{CO}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2

Methyl orange is used as the indicator, turning from yellow to peach/pink at the endpoint.

Materials and Apparatus Required

Chemicals

0.1 M sodium carbonate solution
Hydrochloric acid solution (unknown concentration)
Methyl orange indicator

Apparatus

Burette (50 cm³)
Retort stand and clamp
Conical flask (250 cm³)
Pipette (25 cm³) and pipette filler
Wash bottle
Funnel
White tile
Beakers (250 cm³)
White card
Safety glasses

Safety Precautions

Wear safety glasses at all times.
Handle concentrated hydrochloric acid with care; it is highly corrosive and can irritate the skin and lungs.
Methyl orange contains ethanol and is flammable; keep away from open flames.

Method

Pour approximately 50 cm³ of hydrochloric acid solution into a clean, dry beaker.
Rinse the burette with deionised water, then with a small amount of the hydrochloric acid solution.
Fill the burette with hydrochloric acid solution above the zero mark, remove the funnel, and adjust the level to zero by draining some acid into a beaker.
Rinse the pipette with deionised water, then with the sodium carbonate solution.
Use the pipette to transfer 25 cm³ of sodium carbonate solution to a clean conical flask.
Add 2-3 drops of methyl orange indicator to the conical flask.
Perform a rough titration by adding hydrochloric acid from the burette in 1 cm³ increments, swirling the conical flask continuously, until the indicator changes colour (peach/pink).
Note the volume of hydrochloric acid used and clean the conical flask.
Perform accurate titrations by adding hydrochloric acid rapidly until near the endpoint, then dropwise while swirling until the colour change occurs.
Repeat the titration until two readings are within 0.1 cm³ of each other.
Calculate the concentration of the hydrochloric acid solution.

Results

Measurement	Value
Rough titre	27.0 cm³
Second titre	26.6 cm³
Third titre	26.7 cm³
Average of accurate titres	26.65 cm³
Volume of sodium carbonate solution	25.0 cm³
Concentration of sodium carbonate	0.1 M
Concentration of hydrochloric acid	0.19 M

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Sample Calculation

Using the formula:

\text{Moles of Na}_2\text{CO}_3 = \frac{25.0 \times 0.1}{1000} = :highlight[0.0025] \, \text{moles}

From the balanced equation,

:highlight[2]\ \text{moles HCl react with 1 mole Na}_2\text{CO}_3

Therefore,

\text{Moles of HCl} = 2 \times 0.0025 = :highlight[0.005] \, \text{moles}

The concentration of HCl:

\frac{0.005}{26.65/1000} = :success[0.19] \, \text{M}

Example Questions with Answers

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Q1: Why is a conical flask used instead of a beaker for the titration?

The conical flask allows easy swirling, which promotes thorough mixing of the acid and base during titration.

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Q2: Why is the funnel removed from the burette after filling it with acid?

To prevent any drops of acid from falling into the burette after filling, which would alter the volume of acid added.

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Q3: Why is it important to rinse the burette with hydrochloric acid before use?

Rinsing removes any water that could dilute the acid, ensuring accurate concentration in the titration.

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Q4: Why is a rough titration carried out first?

To get an approximate idea of the endpoint, allowing subsequent titrations to be done more quickly and accurately.

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Q5: Explain why hydrochloric acid is not used as a primary standard.

Hydrochloric acid is not pure enough for use as a primary standard; its exact concentration must be determined using a standard solution.

- Standardisation of a Hydrochloric Acid Solution Simplified Revision Notes for Leaving Cert Chemistry

4.2 - Standardisation of a Hydrochloric Acid Solution

Experiment Summary

Materials and Apparatus Required

Chemicals

Apparatus

Safety Precautions

Method

Results

Sample Calculation

Example Questions with Answers

Q1: Why is a conical flask used instead of a beaker for the titration?

Q2: Why is the funnel removed from the burette after filling it with acid?

Q3: Why is it important to rinse the burette with hydrochloric acid before use?

Q4: Why is a rough titration carried out first?

Q5: Explain why hydrochloric acid is not used as a primary standard.

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