# Why does oxalic acid have exactly two water molecules of crystallization?

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?

• You might want to start by looking at the crystallization structure of oxalic acid bihydrate. The number is exactly $2$. – DHMO Oct 6 '16 at 15:29
• 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. – Ivan Neretin Oct 6 '16 at 15:33
• @Jan the left is with serine; the right without. – DHMO Oct 6 '16 at 23:23
• @DHMO True that … – Jan Oct 6 '16 at 23:25

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: