4.4 - Determination Percentage Water of Crystallisation in a Sample of Hydrated Sodium Carbonate Revision Notes for Leaving Cert Chemistry

4.4 - Determination Percentage Water of Crystallisation in a Sample of Hydrated Sodium Carbonate

infoNote

Experiment Summary

In this experiment, the percentage of water of crystallisation in a sample of hydrated sodium carbonate ( $Na₂CO₃.xH₂O$ ) is determined by titration.

A solution of hydrated sodium carbonate is titrated against a standard solution of hydrochloric acid ( $HCl$ ). The titration is used to calculate the amount of sodium carbonate, allowing the formula of the hydrated compound to be determined.

Materials and Apparatus Required

Chemicals

0.1 M hydrochloric acid
Hydrated sodium carbonate
Methyl orange indicator
Deionised water

Apparatus

Clock glass
Stirring rod
Burette (50 cm³) with retort stand and clamp
Beakers (250 cm³)
Volumetric flask (250 cm³)
Conical flask (250 cm³)
Pipette (25 cm³) and pipette filler
Dropping pipette
Wash bottle
White tile
Safety glasses

Safety Precautions

Wear safety glasses at all times.
Sodium carbonate can irritate the eyes and skin. Avoid contact and use gloves if necessary.
Methyl orange contains ethanol and is flammable; keep away from open flames.

Method

Weigh approximately 1.5 g of hydrated sodium carbonate using a clock glass.
Add about 50 cm³ of deionised water to a beaker and stir to dissolve the sodium carbonate completely.
Transfer the solution to a 250 cm³ volumetric flask, rinsing the beaker with deionised water and adding the rinsings to the flask.
Make up the solution to the 250 cm³ mark with deionised water, using a dropper for the final few drops.
Stopper the flask and invert several times to mix.
Rinse the burette with hydrochloric acid and the pipette with sodium carbonate solution.
Use the pipette to transfer 25 cm³ of sodium carbonate solution to a conical flask.
Add 2-3 drops of methyl orange indicator.
Perform a rough titration by adding hydrochloric acid from the burette while swirling the conical flask, until the solution changes from yellow to peach/pink.
Repeat the titration until two accurate results agree within 0.1 cm³.
Calculate the amount of water of crystallisation in the sample based on the titration results.

Results

Measurement	Value
Mass of hydrated sodium carbonate	1.57 g
Rough titre	13.0 cm³
Second titre	12.5 cm³
Third titre	12.6 cm³
Average of accurate titres	12.55 cm³
Volume of sodium carbonate solution	25.0 cm³
Concentration of hydrochloric acid	0.1 M

infoNote

Sample Calculation

Moles of $HCl$ used:

\frac{12.55 \times 0.1}{1000} = :highlight[0.001255 \, \text{moles}]

From the balanced equation:

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

Moles of $Na₂CO₃$ used:

0.001255 / 2 = :highlight[0.0006275 \, \text{moles}]

Mass of $Na₂CO₃$ in 25 cm³:

0.0006275 \times 106 = :highlight[0.0665 \, \text{g}]

Mass of $Na₂CO₃$ in 250 cm³:

0.0665 \times 10 = :highlight[0.665 \, \text{g}]

Mass of water in $Na₂CO₃.xH₂O$ :

1.57 - 0.665 = :highlight[0.905 \, \text{g}]

Percentage of water in $Na₂CO₃.xH₂O$ :

\frac{0.905}{1.57} \times 100 = :highlight[57.64\%]

Moles of water in $Na₂CO₃.xH₂O$ :

\frac{0.905}{18} = :highlight[0.05028 \, \text{moles}]

Moles of $Na₂CO₃$ :

0.0006275 \times 10 = :highlight[0.006275 \, \text{moles}]

Value of $x$ :

\frac{0.05028}{0.006275} = :highlight[8.01] \Rightarrow \text{Na}_2\text{CO}_3.8\text{H}_2\text{O}

Example Questions with Answers

infoNote

Q1: What was done to the volumetric flask after the solution was made up to the mark?

The flask was stoppered and inverted several times to ensure the solution was homogeneous.

infoNote

Q2: Why is it preferable to use as little indicator as possible in acid-base titrations?

Indicators are weak acids or bases. Adding too much can affect the titre value.

infoNote

Q3: Name suitable apparatus for measuring (i) the sodium carbonate solution and (ii) the hydrochloric acid.

(i) Pipette for sodium carbonate solution.
(ii) Burette for hydrochloric acid.

infoNote

Q4: Explain water of crystallisation.

Water molecules are chemically bound in a crystalline compound, contributing to its structure.

infoNote

Q5: Name another compound with water of crystallisation.

Hydrated copper sulphate ( $CuSO₄.5H₂O$ ).

4.4 - Determination Percentage Water of Crystallisation in a Sample of Hydrated Sodium Carbonate (Leaving Cert Chemistry): Revision Notes