What is the pH of #1.2 * 10^-3# HBr solution?

Even without doing any calculations, you can say that the pH of this solution will be a little lower than #3#.

Why is that the case?

Hydrobromic acid, #"HBr"#, is a strong acid, which means that it ionizes completely in aqueous solution to form hydronium ions, #"H"_3"O"^(+)#, and bromide anions, #"Br"^(-)#.

#"HBr"_text((aq]) + "H"_2"O"_text((l]) -> "H"_3"O"_text((aq])^(+) + "Br"_text((aq])^(-)#

Now, when an acid ionizes completely, you can expect all the molecules to donate their proton to the water. This of course means that the initial concentration of hydrobromic acid will now be equivalent to the concentration of hydronium ions and bromide anions.

Since every molecule of #"HBr"# produces one hydronium ion in aqueous solution, you can say that

#["H"_3"O"^(+)] = ["HBr"] = 1.2 * 10^(-3)"M"#

As you know, the pH of the solution is a measure of the concentration of hydronium ions

#"pH" = -log( ["H"_3"O"^(+)])#

In this case, the pH of the solution will be

#"pH" = - log(1.2 * 10^(-3)) = color(green)(2.92)#

As predicted, the pH is a little lower than #3#. If the initial concentration of hydrobromic acid would have been #1.0 * 10^(-3)"M"#, then the pH would have been equal to #3#.

Since

#1.2 * 10^(-3)"M" > 1.0 * 10^(-3)"M"#

the pH will be lower than #3#, i.e. the solution is more acidic.