4: To determine the amount of water of crystallisation in washing soda crystals (hydrated sodium carbonate)

Theory

• Washing soda crystals contain hydrated sodium carbonate Na2CO3·XH2O

• By preparing a solution of washing soda and titrating it against a previously standardised solution of hydrochloric acid (HCl), the amount of water of crystallisation can be determined and the therefore the value of X in Na2CO3·XH2 can be found

Equation

2HCl + Na2CO3 → 2NaCl + H2O + CO2

Procedure

1) Filling the burette with the standard solution of hydrochloric acid solution

2) Making the washing soda (hydrated sodium carbonate) up into a solution

3) Transferring an exact volume of washing soda solution into a conical flask

4) Carrying out the titration

Suitable indictor for this titration

Methyl orange

Justification for this indicator

• It is a strong acid-weak base titration, the end point occurs below a pH of 7

• Methyl orange changes colour between pH 3 and 5 coinciding with this end point

Colour change observed at end point

Yellow → pink

(Base) → (acid)

How was it possible to have a standard solution of hydrochloric acid to use in this titration despite the fact it is not a primary standard?

The hydrochloric acid was previously standardised by titrating it against a standard solution of anhydrous sodium carbonate – a primary standard (Titration 1)

Explain why a standard solution of hydrated sodium carbonate cannot be directly made up from the washing soda crystals/Explain why a titration is required to determine the amount of water of crystallisation in washing soda crystals

• Washing soda crystals are not a primary standard - they are hydrated sodium carbonate crystals; the crystals lose some of their water of crystallisation (effloresce) in dry air – cannot be obtained in a pure state

• Therefore, a precise mass of these crystals cannot be weighed out and a standard solution cannot be directly made up

Explain the difference between a hydrated and an anhydrous compound

• A hydrated compound has molecules of water attached in the ionic crystal

• An anhydrous compound does not have molecules of water attached in the ionic crystal

Note: Heating a hydrated compound can cause it to lose its water and become anhydrous

Give two uses of washing soda crystals

1) To soften water i.e. remove hardness from water

2) To clean clothes