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).

04:56

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).

In the first part of this question, we’re being asked to compare the hydrogen ion concentration of two acids, formic acid and hydrobromic acid. Let’s recall that when you introduce an acid to water, it will break up to form hydrogen ions and an anion. There are a couple of factors that can change the concentration of the hydrogen ions in our acidic solution; first of all is the concentration of our acid. Recall that concentration is defined as moles per liter. So if we have a higher concentration of our acid, that means that there is a larger amount of our acid that can dissociate to form hydrogen ions, which will lead to a larger concentration of hydrogen ions in the solution.

The second factor is the strength of our acid. Acids can be strong or they can be weak. This means that if we introduce two different acids to water, one that’s a strong acid and one that’s a weak acid, the strong acid solution will end up having a higher concentration of hydrogen ions because all of the acid molecules will dissociate to form hydrogen ions, whereas in the weak acid solution, some of the molecules will remain intact. So there’ll be fewer hydrogen ions in the solution.

In statement (I) of this question, we’re told that the concentrations of both formic acid and hydrobromic acid are the same. They’re both 0.5 molar. So, that means we need to compare the strength of these acids to determine which one will have a higher concentration of hydrogen ions. There’s unfortunately no trick to knowing which acids are strong and which ones are weak. This is something that needs to be memorized, but there’s only seven strong acids. So once you know what they are, you can determine that everything else must be a weak acid.

Our list of strong acids includes HCl or hydrochloric acid, HBr or hydrobromic acid, HI or hydroiodic acid, as well as HClO4 which is called perchloric acid, H2SO4 sulfuric acid, HNO3 or nitric acid, and HClO3 chloric acid. However, you occasionally won’t see chloric acid counted on the list of strong acids. So as we can see, this question is asking about hydrobromic acid and formic acid. And hydrobromic acid is clearly on our list of strong acids. So since hydrobromic acid is a strong acid and formic acid is a weak acid, the concentration of hydrogen ions in the formic acid solution will be smaller than the concentration of hydrogen ions in the hydrobromic acid solution. So statement (I) is true.

Our next statement is asking us to figure out which one of our molecules contains more atoms, formic acid or hydrogen bromide? You’ll notice that the first part of this question referred to HBr as hydrobromic acid, but in the second part of the question, it just says hydrogen bromide. This is because HBr is only referred to as hydrobromic acid when it’s dissolved in water. Otherwise, we refer to it as hydrogen bromide. But we’re still talking about the same molecule.

So to answer this part of the question, let’s compare the structures for hydrogen bromide and formic acid. We can see that hydrogen bromide is just composed of two atoms, a hydrogen and bromine. Formic acid, on the other hand, is composed of five atoms. So statement (II) is clearly true as well. A molecule of formic acid does contain more atoms than a molecule of hydrogen bromide.

Since both statement (I) and (II) are true, we have to state if (II) is a correct explanation for (I). That is, we need to state if the reason that the concentration of hydrogen ions is smaller in formic acid than it is in hydrobromic acid is because a molecule of formic acid contains more atoms. As we know from our previous discussion, this is clearly false. The concentration of hydrogen ions is smaller in our solution of formic acid not because of the amount of atoms that formic acid has, but because formic acid is a weak acid, while hydrobromic acid is a strong acid.

Nagwa uses cookies to ensure you get the best experience on our website. Learn more about our Privacy Policy.