For statements I and II, state for each if they’re true or false. I) Oxalic acid, C₂H₂O₄, is a stronger acid than hydrochloric acid. II) Oxalic acid is a diprotic acid, while hydrochloric acid is a monoprotic acid. If both are true, state if II is a correct explanation for I.
Oxalic acid is an organic acid. Oxalic acid occurs naturally in plants such as broccoli, spinach, cabbage, and other leafy greens. Oxalic acid is what’s known as a dicarboxylic acid because it’s composed of two carboxyl groups. Acids that are composed of these carboxyl groups are weak acids. When weak acids are introduced to a solution, they don’t completely dissociate or break up into ions. We can compare this to a strong acid, which will completely dissociate or break up into ions when introduced to a solution.
Since how acidic a solution is is related to the concentration of hydrogen ions in the solution, if we have a weak acid solution and a strong acid solution of the same concentration, the strong acid solution will be more acidic because more of the acid molecules dissociate to form H⁺ ions. There’s unfortunately no trick to help you remember what acids are strong acids and which acids are weak acids. So it’s something that you need to memorize. But there’s fortunately only a small number of strong acids. So let’s go over what they are.
The strong acids are HCl, or hydrochloric acid; HBr, hydrobromic acid; HI, which is hydroiodic acid; HNO₃, nitric acid; H₂SO₄, which is sulfuric acid; HClO₄, which is perchloric acid; and HClO₃, which is chloric acid. Some textbooks and teachers don’t list HClO₃ as a strong acid. But the other six acids are universally accepted as strong acids.
Statement I says that oxalic acid is a stronger acid than hydrochloric acid. As we can see from our list, HCl or hydrochloric acid is a strong acid. But oxalic acid, as we’ve discussed, is a weak acid. So statement I is false. Oxalic acid is in fact a weaker acid than hydrochloric acid is.
Statement II says that oxalic acid is a diprotic acid, while hydrochloric acid is a monoprotic acid. A monoprotic acid is an acid that when it dissociates will give one H⁺ ion to the solution. It’s called monoprotic because an H⁺ ion contains no electrons, only a single proton. So a monoprotic acid gives one proton to the solution. Diprotic acids, on the other hand, will dissociate into two H⁺ ions. Since hydrochloric acid has the chemical formula HCl, we can see that it only has one hydrogen. Since it only has one hydrogen, it’s a monoprotic acid.
Looking at the structure of oxalic acid, we can see that it does have two hydrogens. So it’s a diprotic acid. We should be careful here when determining whether or not an acid is diprotic or even triprotic because not all hydrogens that are in an acid necessarily will dissociate when the acid enters the solution. In acetic acid, for example, which is another carboxylic acid, only the hydrogen that’s attached to the oxygen will dissociate. The other three hydrogens that are bonded to the carbon will not. So even though acetic acid has four hydrogens in its structure, acetic acid is still just a monoprotic acid.
Either way, statement II is true. Oxalic acid is a diprotic acid, while hydrochloric acid is a monoprotic acid. Since statement I is false, we don’t have to answer the last part of the question.