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Chem. Practical Task :)

This time it's educational, since I've been given the practical planning paper for Chemistry OCR.


I have to try to find out the number of moles of water of crystallisation in a mole of hydrated sodium carbonate by heating the crystals. I have no idea how to do this experiment or how to work out the moles from the results. Any help would be greatly appreciated!

Jenkin replies: I will suggest an outline for this task but you will need to think out some of the practical details yourself. Essentially, you need to take a known mass of the crystals and heat them to drive off the water of crystallisation. A crucible or small evaporating dish would be a suitable vessel. Allow to cool and then reweigh. To ensure that all the water has been driven off you will need to heat again, allow to cool and weigh again, this to be repeated until the weight is constant. The calculation can be along these lines:

Mass of hydrated crystals: a grams

Mass of water of crystallisation (loss in mass of crystals): b

Mass of anhydrous sodium carbonate: (a – b)

Moles of anhydrous sodium carbonate, Na2CO3: (a – b) / molar mass: call this
d moles of Na2CO3)

Moles of H2O: b / molar mass: call this e moles of H2O

So now we know that d moles of Na2CO3 are combined with e moles of H2O and , therefore, that 1 mole of Na2CO3 will be combined with d / e moles of H2O.

The second part is titration, which I am still trying to research out. So far, no luck.. Going to look into my Chemistry Bible later for some enlightenment.

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