Why is Cu(NO3)2 Acidic?

[Megan Chan 3A]

Question 6D.11 from the textbook asks: Decide whether an aqueous solution of each of the following salts has a pH equal to, greater than, or less than 7.

The answer key says that Cu(NO3)2 has a pH less than 7. How do we determine that?

[Stuti Pradhan 2J]

In Cu(NO3)2, the NO3- is the conjugate base of a strong acid (HNO3), so it will not affect the pH of the aqueous solution. However, Cu2+ is not the conjugate acid of a strong base, so it will affect the pH. Since Cu2+ has a positive charge, it will attract the oxygen atom from the water molecule. This will cause the bonds in water to weaken and one of the H-O bonds will break, leaving an H+ ion in the aqueous solution, which will decrease the pH.

Hope this helps!

[Scot Widjaja Dis 1J]

The general rule of thumb is that transition metals that are highly positive charged like in the case of Cu2+ above will generally cause the pH to decrease less than 7 when dissolved in an aqueous solutions. The reason why the nitrate ion doesn’t affect pH is because it’s a conjugate base of a strong acid, HNO3. Only conjugate bases of weak acids like CH3COO- will increase the pH when dissolved in an aqueous solution.