Why is Cu(NO3)2 Acidic?

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Megan Chan 3A
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Joined: Wed Sep 30, 2020 9:50 pm

Why is Cu(NO3)2 Acidic?

Postby Megan Chan 3A » Thu Dec 10, 2020 10:07 pm

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
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Re: Why is Cu(NO3)2 Acidic?

Postby Stuti Pradhan 2J » Thu Dec 10, 2020 10:28 pm

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
Posts: 75
Joined: Wed Sep 30, 2020 9:55 pm

Re: Why is Cu(NO3)2 Acidic?

Postby Scot Widjaja Dis 1J » Sun Jan 17, 2021 11:49 pm

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.


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