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Why does oxalic acid have exactly two water molecules of crystallization?
In other words, in the formula $\ce{C2H2O4\cdot2H2O}$, why is the number before the $\ce{H2O}$ exactly $2$?

I tried to see this from the structure of oxalic acid. Also, I tried to invoke hydrogen bonds.
However, I don't see the reason why there are two water molecules of crystallization from this.

Also, with how many precision is it two?

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    $\begingroup$ You might want to start by looking at the crystallization structure of oxalic acid bihydrate. The number is exactly $2$. $\endgroup$ – DHMO Oct 6 '16 at 15:29
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    $\begingroup$ It is 2 because of the crystal structure. And crystal structure, mind you, is not something you may infer just by looking at the molecule, or by some simple back-of-the-envelope estimations. $\endgroup$ – Ivan Neretin Oct 6 '16 at 15:33
  • $\begingroup$ @Jan the left is with serine; the right without. $\endgroup$ – DHMO Oct 6 '16 at 23:23
  • $\begingroup$ @DHMO True that … $\endgroup$ – Jan Oct 6 '16 at 23:25
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As @Ivan Neretin states, the crystal structure determines how much water can be accommodated. For example, $\ce{CuSO4}$, cupric sulfate or copper(II) sulfate, may be found as a mineral with three, five or seven molecules of bound water:

  • Bonattite (trihydrate mineral)
  • Bluestone (pentahydrate)
  • Boothite (heptahydrate mineral)

Of course, it can also be heated to form the anhydrous salt.

One could say different crystal forms have room for different amounts of water, or the converse: by adding or removing water, the crystal structure is changed.

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