How do we tell if it is acidic?

[Garmani Kyaw]

Given the formula, such as Cr(ClO4)3, how do we tell if it is acidic or not?

[Anirudh Mahadev 1G]

Since ClO4- is a conjugate base of a strong acid (very strong) it will be inactive in solution. Since Cr 3+ is not a conjugate of any strong acids or bases, you know it has to be from a weak base because the other component of the salt came from an acid. Therefore the Cr3+ is a conjugate acid of a weak base and would lower the pH of the solution, making it acidic.

[Bella Wachter 1A]

We look at the ions produced by the salt. Here are some rules I like to keep in mind:

1) Conjugate base of a strong acid (anion) = neutral

2) Conjugate acid of a strong base (cation) = neutral

3) Conjugate base of a weak acid (anion) = basic

4) Conjugate acid of a weak base (cation) = acidic

Therefore, for Cr(ClO4)3:

Cr3+ is the conjugate acid of a weak base (Cr(OH)3) - it is an acidic cation

ClO4- is the conjugate base of a strong acid (HClO4) - it is a neutral anion

Therefore, a solution of Cr(ClO4)3 is acidic.

[Keeryth Sandhu 1H]

You have to look at the ions. The conjugate acids of a strong base and the conjugate base of a strong acid both do not affect the pH. The conjugate base of a weak acid (anion) is basic, and vice versa for the conjugate acid of a weak base.