Experiment: To determine the percentage of water of crystallization in hydrated sodium carbonate (washing soda).
Water of crystallization is the water which is found as part of the structure of a crystalline substance. It has nothing to do with being wet. The water molecules referred to in the term occupy positions in the crystal lattice of the substance. This water of crystallization is generally represented in the chemical equations of such compounds, at the end of the formula e.g. Na2CO3.xH2O
The ‘x’ here is a number which represents the number of molecules of water in the crystal. The purpose of this experiment is to determine the percentage of water of crystallization in a substance by titration.
Procedure
- Weigh out accurately 5 g of hydrated sodium carbonate on a clock glass.
- Make up the solution to 250 ml in a volumetric flask. Follow the same procedure as for ‘making a standard solution’ previously outlined above.
- Pipette about 25 ml of this solution into a clean conical flask.
- Add a few drops of methyl red indicator, enough to impart a faint yellow colour to the solution in the conical flask.
- Place 0.2 M HCl in the burette and adjust the level to zero taking all the usual precautions.
- Titrate in the usual manner until the yellow colour is replaced by a permanent pink tinge. This is the end-point of the titration.
- Record the volume of acid required to reach the end-point and repeat several times until two readings (titres) agree to within 0.2 ml of each other.
Results
- Volume of acid used = 23.5 ml
- Factor for the acid = 2
- Molarity of the acid = 0.2 M
- Volume of base = 25 ml
- Factor for the base = 1
- Molarity of base = ?
Calculations
