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MCAT General Chemistry: 3 Dangerous Misconceptions about Acids and Bases

  • by Sam
  • Sep 20, 2016
  • MCAT Blog, MCAT Chemistry, MCAT Prep

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 H2O. (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: Ka of Species A ∙ Kb of Species A = Kw. 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 Ka of H2PO4– and the Kb of H2PO4– is equal to Kw.

Unfortunately, you would be wrong! The Ka of an acid times the Kb of its conjugate base is equal to Kw. Here, this means that Ka (H2PO4-) × Kb (HPO42-) = Kw. When we think about it, this makes perfect sense! After all, if “Acid A” is very strong, it will have a large Ka. 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 Ka (HCl) × Kb (HCl) = Kw. It is the conjugate species, Cl-, which we mean to compare, and which will have a very low Kb value.

Myth #3: Kw 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 Ka, Kb, and Kw. As equilibrium constants, these values vary with temperature. So Ka × Kb 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 Kw = [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 Ka ∙ Kb = Kw, which, for water, can be translated into pH + pOH = pKw. (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!

Written by Blueprint MCAT (formerly Next Step Test Prep) MCAT experts.

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