Chemistry - crystallisation(2)

Hi, let me try!

As you know, water do exist in crystals of compounds, and they are call water of crstallization.
However, one compound may have more than one no. of water of crystallisation
For example, sodium carbonate can be anhydrous, monohydrate(with 1 water of crystallization) and decahydrate(with 10 water of crystallization)

Besides, In the exam question, it ususally requires you to calculate the number base on some experimental results, like acid-base titration.

So, momorizing one compound's number of water of crystallisation, is useless.

In this case, you may find the number of water of crystallisation by using information of mole ratio, formula mass and etc.

Here is an example:
25.025g hydrated sodium carbonate crystals dissolved in 250cm3 of water. 10cm3 of the solution need 17.5cm3 0.2M H2SO4 for complete reaction.

By H2SO4(aq) + Na2CO3(aq) ---> Na2SO4 + H2O + CO2
no. of mole of H2SO4 : no. of mole of Na2CO3
= 1:1
= 0.0035 : 0.0035
no. of mole of hydrated Na2CO3 in the 250cm3 solution
= 0.0875
Formula mass of hydrated Na2CO3
= 25.025 / 0.0875
= 286
No. of water of crystallisation
= [286 - (23/2 + 12 + 16*3)] / 18
= 10

This is the most simple type of question.
Some question may be more complicated. But, to find the no. of water of crystallisation, use similar calculation is OK!


And, of course, you can obtain a pure sample of hydrated compound; say copper sulphate, if you can obatin its pure hydrated crystal.
You can simply weigh it and then heat it until it become anhyrdruos and then weight the anhyrous crystal again immediately.
Find the mass of water evaporated, and then find the mole ratio beetween the anhydrous compound and water. Thus, you can know the no. of water of crystallisation (for CuSO4 is 5).


Hope I can help you !~

2011-07-31 23:23:25 補充：
Thanks 己式庚辛,
X-ray crystallography, of course, is the most suitable way, I guess!
You can even determine the atom arrangement within the crystal.

The method is to strike the crystal by a beam of X-ray,
the beam of X-ray will diffract to different direction,

2011-07-31 23:23:31 補充：
from the angle and intensity of the diffracted rays,
density of electrons in the crystal can be known,
thus, the position and the way that the atoms are bonded can be known.

it could be hard to know whether the crystals are hydrated, or just moist / wet.

let's perform an X-ray crystallography!