# Do we consider the dissociation of water when dealing with ion concentrations of a solution?

One mole of hydrated copper(II) sulfate, $$\ce{CuSO4 . 5 H2O},$$ is dissolved in water. How many moles of ions does the solution contain?

1) 1
2) 2
3) 6
4) 7

The correct answer is 2. Specifically, I'm confused about why the $$\ce{OH-}$$ and $$\ce{H+}$$ ions aren't considered.

• The solution will remain neutral with pH=$7$, thus the concentration of OH$^-$ and H$^+$ will be $10^{-7}$ M which is negligible compared to the copper and sulphate of one molar each. – porphyrin Jun 10 '19 at 10:13

It dissociates into 7 parts; copper and sulfate ions and five moles of water.

Water is also the solvent, so these don't count.

However the answer "two" is indeed not perfectly right, because the sulfate ion would (partially) react with water, to form

$$\ce{SO4^2- + H2O <=> HSO4- + OH-}$$

A sulfate ion is so to say the opposite of sulfuric acid, which reacts

$$\ce{H2SO4 + 2 H2O -> HSO4- + H3O+ + H2O <=> SO4^2- + 2 H3O+}$$

$$\ce{HSO4-}$$ is not strongly acidic anymore $$(\mathrm{p}K_\mathrm{a} = 2),$$ so the sulfate ion logically must react somewhat basic. (OK, $$\mathrm{p}K_\mathrm{a} = 2$$ is quite acidic, acetic acid has $$\mathrm{p}K_\mathrm{a} = 4.8.)$$

So the correct answer is slightly larger than two. But from the given possibilities in this multiple-choice test, two is by far the most reasonable.

(Update: The hydrated $$\ce{Cu^2+}$$ ion might well be acidic enough to overcompensate the basicity of the sulfate. In which case you'd still have more than two equivalents of ions, just it'd be a small amount of $$\ce{H3O^+},$$ not $$\ce{OH^-}.$$ And of course there is always the autoprotolysis of water.)

• @Karl: Do you mean $\ce{Cu^{2+}}$ ions? And also 7 parts, copper ..? – Mathew Mahindaratne May 13 '18 at 20:35

there is no link with dissociation of water, here the salt is following Werner's theory where copper forms a coordinated complex with 4 water molecules while one is hydrogen bonded with $\ce{SO4^2-}$ anion. Concept of water of crystallization is involved here. Upon crystallization from water or moist solvents, many compounds incorporate water molecules in their crystalline frameworks. Here is the formula of the copper salt in its hydrated form $\ce{[Cu(H2O)4]SO4·H2O}$. Coordinated compounds don't break down easily so copper with 4 water molecules inside the coordination sphere remains intact and does not dissociate when put in water. The salt dissociates as $\ce{[Cu(H2O)4]2+}$ and $\ce{SO4^2-}$ anion and hence 2 ions in water.