If you’ve taken the MCAT before, or if you’re in the midst of the MCAT prep process, you probably understand the importance of acid-base chemistry. This topic is one of the highest-yield of any in general chemistry, and perhaps more importantly, it relates to amino acids, organic chem, and more. But watch out! Even the most seasoned MCAT students often fall victim to mistaken ideas about acids and bases. Here are 3 misconceptions that we hope will never trip you up again:

Myth #1: pH ranges from 0 to 14.

Many students firmly believe this, but it is entirely false! Remember, pH is equal to the negative logarithm of the hydrogen ion concentration of a solution. Consider this example: a student adds 10 moles of HBr to 0.1 L of distilled water. As a strong acid, HBr completely dissociates, so we now have 10 moles of H⁺ in 0.1 L H_²O. (10 moles) / (0.1 L) = 100 M H⁺.

Next, find the negative log of this value. 100 is equal to 10², so let’s find -log(10²). Log(10²) = 2, so -log(10²) = -2. We see that in this highly concentrated solution, the pH is actually negative! Note that pH can also exceed 14 in highly basic solutions.

Myth #2: K_a of Species A ∙ K_b of Species A = K_w.

This myth appears most often in reference to polyprotic acids, so let’s consider H2PO4-, which is the conjugate base of phosphoric acid. If you fall victim to this myth, you may think that the product of the K_a of H₂PO₄– and the K_b of H₂PO₄– is equal to K_w.

Unfortunately, you would be wrong! The K_a of an acid times the K_b of its conjugate base is equal to K_w. Here, this means that K_a (H₂PO₄-) × Kb (HPO₄^2-) = K_w. When we think about it, this makes perfect sense! After all, if “Acid A” is very strong, it will have a large K_a. The conjugate of a strong acid is always a very weak base, so the conjugate of “Acid A” will have a small Kb. In fact, with monoprotic acids (HCl, HI, etc.) this myth is far less tempting than in the previous situation with H₂PO₄-. Consider HCl – of course we don’t mean to say that K_a (HCl) × K_b (HCl) = K_w. It is the conjugate species, Cl-, which we mean to compare, and which will have a very low K_b value.

Myth #3: K_w is always 10^-14, and pH and pOH always add up to 14.

As a tutor, I can’t tell you how many students believe this myth, or one of its many variations. Have you ever thought that a pH of 7 was always neutral, or that a pH of 6.5 was always acidic? Then you’ve fallen for this myth! Acid-base math is dictated by the equilibrium constants K_a, K_b, and K_w. As equilibrium constants, these values vary with temperature. So K_a × K_b does equal 10^-14, but only at standard temperature (25°C)!

Let’s look at an example. At 60°C, the warm temperature causes water to dissociate more than it would at 25°C. Since K_w = [H₃O⁺][OH^–], and since more hydronium and hydroxide ions are present at 60°C, Kw is much larger than 10^-14. In fact, it is approximately equal to 10^-13. But what does this mean? Well, remember that K_a ∙ K_b = K_w, which, for water, can be translated into pH + pOH = pK_w. (We won’t do the derivation here, but feel free to try it on your own!) Importantly, this means that pH + pOH = 13 at 60°C.

If pH + pOH = 13, then what is the pH of a glass of 60°C water? Here’s all you need to remember: in any neutral solution, pH = pOH. Here, pH = 6.5, while pOH = 6.5 as well. A pH of 6.5 may sound acidic, and pH 6.5 water certainly does have more free H3O+ ions than water at pH 7! But if the concentration of hydroxide ions is equally high, the solution must be neutral.

We hope you have enjoyed these tidbits of general chemistry knowledge, and wish you good luck on your MCAT journey! For more free resources like a practice test, study planner and 1600+ flashcards, get our free MCAT account!