Is an aqueous solution of Cu(NO3)2 acidic, basic or neutral?

Question

i dont understand why it would be acidic because HNO3 is a strong acid so its conjugate would be a base (NO3) right?

Answer 1

Because HNO3 is a strong acid, NO3- will be an extremely weak base - neutral for all practical purposes.

I don't think that Cu2+ is the conjugate acid of a weak base - if so, which one?

Anyway, Cu2+ does not exist in solution as such: it exists as a complex ion with water molecules as ligands - [Cu(H2O)4]2+

The hydroxide ion can also act as a ligand, but H+ cannot (no unshared pairs), so the complex ion can react as follows:

[Cu(H2O)4]2+ ----> [Cu(H2O)3(OH)]+ + H+

(more waters can also dissociate in this way)

And the solution becomes acidic.  All transition metal ions in water do this to varying degrees.

Answer 2

Cu(NO3)2 ~~> Cu(2+) + 2NO3(1-)

NO3(1-) is the conjugate base of a strong acid HNO3, which makes it a weak base.

Cu(2+) is the conjugate acid of a weak base, which makes it acidic and since it dissociates more than NO3(1-) the solution itself is acidic.