Video: Applying Knowledge of Acid Strength and Composition

For statements I and II, state for each if they are true or false. I) [H⁺] is smaller in 0.5 M formic acid (methanoic acid) than in 0.5 M hydrobromic acid. II) A molecule of formic acid (methanoic acid) contains more atoms than a molecule of hydrogen bromide. If both are true, state if II is a correct explanation for I.

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Video Transcript

For statements I and II, state for each if they are true or false. I) The concentration of H⁺ is smaller in 0.5-molar formic acid or methanoic acid than in 0.5-molar hydrobromic acid. II) A molecule of formic acid or methanoic acid contains more atoms than a molecule of hydrogen bromide. If both are true, state if II is a correct explanation for I.

When you put an acid into water, it dissociates and breaks up into hydrogen ions and an anion. If the acid fully dissociates, we call it a strong acid. If it doesn’t fully disassociate, we call it a weak acid. Because strong acids fully disassociate, they’ll produce more H⁺ ions in water. So if we had, for example, a one-molar solution of a strong acid and a one-molar solution of a weak acid, the strong acid solution would have a higher concentration of H⁺ ions because it dissociates more. And so it produces more H⁺ ions than the weak acid does.

So how do you know if an acid is a strong acid? Well, unfortunately, this is something that you have to memorize. But the number of them is small, so let’s go through all of them. The strong acids are HCl, or hydrochloric acid; HBr, hydrobromic acid; HI, hydroiodic acid; HNO₃, nitric acid; H₂SO₄, which is sulfuric acid; HClO₄, which is perchloric acid; and HClO₃, which is chloric acid. Some instructors and textbooks don’t list chloric acid as a strong acid. But the remaining six are consistently listed as strong acids.

Now that we understand the difference between strong and weak acids, let’s take a look at statement I. Statement I is asking us if the concentration of H⁺ ions will be smaller in a 0.5-molar solution of formic acid than the same-concentration solution of hydrobromic acid. Hydrobromic acid, or HBr, is one of the acids on our list, which means it’s a strong acid. But formic acid, also known as methanoic acid, is not, which means it’s a weak acid. Both of these acids have a concentration of 0.5 molar. But one is strong and one is weak.

Since strong acids fully dissociate in water, the strong acid will have a higher concentration of H⁺ ions. So statement I is true. The concentration of H⁺ ions will be smaller in the formic acid solution than in the hydrobromic acid solution because formic acid is a weak acid.

Now let’s take a look at statement II. Statement II wants to know if a molecule of formic acid contains more atoms than a molecule of hydrogen bromide. This is what formic acid looks like. Formic acid is what’s known as a carboxylic acid because it contains this group that contains a carbon, which is double-bonded to an oxygen and also single-bonded to another oxygen, which is bonded to a hydrogen.

When formic acid dissociates in water, it’s the bond between the hydrogen and the oxygen that breaks. So the hydrogen that’s attached to the oxygen is the one that produces the hydrogen ions and not the one that attach to the carbon. This is what hydrobromic acid or hydrogen bromide looks like, just a hydrogen and a bromine. Formic acid contains five atoms, where hydrobromic acid contains two atoms. So statement II is true. A molecule of formic acid contains more atoms than a molecule of hydrobromic acid.

Since statement I and II are both true, we need to state if II is a correct explanation for I. We’ve already discussed how the concentration of hydrogen ions will be smaller in formic acid than in hydrobromic acid because hydrobromic acid is a strong acid, so it fully dissociates, where formic acid doesn’t. This has nothing to do with how many atoms formic acid has, as suggested in statement II. So it’s false that statement II is a correct explanation for I.

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