# Why is the solubility of sodium acetate trihydrate so much lower than anhydrous sodium acetate?

According to Wikipedia:

Solubility of anhydrous sodium acetate in water at 0 $^{\circ}$C is 119 g/100 mL (AKA 1.495 mol/100mL)

Solubility of trihydrate sodium acetate in water at 0 $^{\circ}$C is 36.2 g/100 mL.

This makes no sense to me because it should take MORE of the trihydrate sodium acetate (about 1.495 * 136.08 g) to dissolve the same amount of sodium acetate.

So this begs two questions:

1.) Is my understanding of hydrates incorrect? Are their bonds somehow harder to solvate than the anhydrous version? I think this can't be correct because the melting point of sodium acetate trihydrate is 58 $^{\circ}$C compared to the anhydrous melting point of 324 $^{\circ}$C.

2.) Is my understanding of solubility incorrect? I thought solubility is a measure of the quantity of solute that could be dissolved in a solvent. In other words, it seems that MORE sodium cations and acetate anions can be dissolved in 100 mL of water if they came from an anhydrous crystal rather than a trihydrate crystal... How can that be true?

• See fr.wikipedia.org/wiki/Ac%C3%A9tate_de_sodium there's 362 not 36.2 - someone put decimal point in wrong place. – Mithoron Aug 7 '16 at 22:27
• The figures in the English Wikipedia for the solubility on anhydrous sodium acetate in water are most probably wrong. – aventurin Aug 7 '16 at 23:09
• @Mithoron That still doesn't make sense because that is literally three times the amount than anhydrous can be dissolved...@aventurin I can't seem to find anywhere another source of the solubility of anhydrous sodium acetate – Nova Aug 8 '16 at 0:51
• The trihydrate already has it's water of crystallisation in the crystal lattice so it would take longer to dissociate in water , it would be less hygroscopic . – Technetium Aug 8 '16 at 2:08
• @Mithoron The values in French Wikipedia are given per liter, so they're the same. – vapid Aug 8 '16 at 9:40

The values of solubility for these salts are $100\%$ correct. Your understanding of solubility is also correct. What you didn't know (probably) is that solutions can exist in a non-equilibrium state. In case of anhydrous sodium acetate, it dissolves in water forming so-called supersaturated solution, which is metastable. Eventually, crystals of sodium acetate trihydrate will precipitate out of the solution, forming a system which is in the thermodynamic equilibrium.