Acid base and pH

Question

Determine the pH of a 6 ×10^−4 M solution of HBr.

Answer 1

Hi Shaun!

When determining the pH of a substance the first thing you want to do is determine whether a.) you are working with an acid or a base and b.) whether that acid/base is strong or weak.

In this problem we are dealing with HBr. Immediately we can identify this as an acid because acids generally have an H out in front. Now we have to see whether this is a strong acid or a weak acid. Unfortunately, the best way to distinguish a strong acid from a weak acid is just to memorize all of the strong acids :(

Luckily, there are only about 7 strong acids, and HBr is one of them! :)

Since HBr is a strong acid, we know that it must completely dissociate into its individual ions in solution. This is the definition of a strong acid. So now, we have both H⁺ ions and Br^- ions in solution. We can see that for every one mole of HBr, there will be one mole H⁺ ions, so the concentration of HBr is equal to the concentration of H⁺.

Now, all we have to do is plug our values into the pH equation:

pH = -log [H^+]

pH = -log [6.0 x 10⁴ M]

pH = 3.22

We can check if our answer makes sense by evaluating the value we got for our pH. We know that the pH of acids must be <7 and the pH of strong acids should be significantly lower. We got a pH of 3.22 which indicates a relatively strong acid like HBr in this problem.