Determination of the water of crystallisation in washing soda crystals.

Washing soda is nominally Na₂CO₃.10H₂O, but it loses water to the atmosphere (effloresces), developing a white powdery surface to the crystals. The material is Na₂CO₃.xH₂O where x will be less than 10 by an amount depending on the age of the crystals – and indeed varying from crystal to crystal.

Standard hydrochloric acid soluiton is used to titrate the sodium carbonate solution:

Na₂CO₃ + 2HCl à 2NaCl + H₂O + CO₂

1 Weigh accurately about 3.58 g of washing soda crystals, and dissolve them in about 50 cm³ of pure water in a beaker. Transfer this solution completely to a 250 cm³ graduated flask, make to the mark with pure water, and mix well.

2 Fill a burette with 0.10 mol dm^-3 hydrochloric acid (which has previously been standardised against, for example, anhydrous sodium carbonate).

3 Pipette 25.0 cm³ of the washing soda solution into a 250 cm³ conical flask, add 3 – 4 drops of methyl orange (or screened methyl orange) indicator, and titrate with standard hydrochloric acid until the solution just turns pink-red.

4 Repeat to obtain three consistent titres.

Results:

Mass of washing soda used/g:

Mean titre/cm³:

Calculation.

You will know the amount (moles, remember) of HCl you used, and therefore the amount of sodium carbonate present in your crystals. Since you also know how much you weighed out you can find the molar mass of your crystals. Anhydrous sodium carbonate has a molar mass of 106 g, so anything over this is due to water of crystallisation. Divide by 18 g mol^-1 to find the amount of water of crystallisation.

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