(3.b) 0.050 M

[H⁺] is not equal to 1/2 [HAc]. When a molecule is in solution, you do not end up with one-half as much of each kind of ion, but rather you get equal amounts of each ion. In this case, HAc is a weak acid and does not completely dissociate. You will need to look up a value for Ka (acid dissociation constant for acetic acid).