If a 0.1 molar aqueous solution of each of the following compounds was made, which compound would produce the solution with the highest concentration of hydronium ions? a) CH₃COOH, b) NaCl, c) Ba(OH)₂, or d) KBr.
First, let’s work out what the question is really asking us. We’re being asked about the production of hydronium ions. These can be written H₃O⁺. Hydronium ions are produced by the protonation of a water molecule. In order to protonate a water molecule, we’re going to need a source of H⁺. So let’s consider where we might get a source of H⁺. You should remember that lots of acids are good sources of H⁺ ions. A strong acid means an acid which fully dissociates in water. So a strong acid produces lots of H⁺. A weaker acid would only partially dissociate, which still produces some H⁺ but not as much as a strong acid.
Remember that strong and weak acids have nothing to do with the concentration that they’re at. In our question, we’re told that each of our compounds are at the same concentration, 0.1 molar. So what this question is really asking us is which of these compounds is an acid? Moreover, if there’s more than one acid on the list, we’re looking for the strongest acid, since that will produce the most hydronium ions. So let’s look at each one in turn. a, CH₃COOH is also called ethanoic acid or sometimes acetic acid. This is what you find in vinegar. When you add ethanoic acid to water, it dissociates forming H⁺ ions and an acetate anion. We can see that we have produced H⁺, which means that we will be producing hydronium ions. So this is a good candidate for the correct answer.
Compound b, NaCl, sodium chloride, is table salt; the salt that you put on food. When you add this salt to water, it will dissociate into sodium cations and chloride anions. You can see that we haven’t produced H⁺ here. And if you measure the pH of sodium chloride in water, it doesn’t really change much. So sodium chloride is not a good source of H⁺ and doesn’t change the pH. So this is not the right answer. Remember that the pH of an acid is a low number, numbers less than seven.
Compound c is barium hydroxide. When we add this to water, it dissociates into barium cations and hydroxide anions. You should know that hydroxide ions are the opposite of acidic. They are in fact basic. So this will raise the pH. Therefore, this will not produce H⁺ and not produce hydronium ions. So this cannot be the correct answer.
Finally, we come to compound d, potassium bromide. When added to water, potassium bromide dissociates into potassium cations and bromide anions. As you can see, this doesn’t produce H⁺. So it won’t produce hydronium ions. And if you were to measure the pH, it wouldn’t really change. It would stay about seven. So again, this is not the correct answer.
So looking down our list, the only acid is compound a, ethanoic acid. Acids are a good source of H⁺ which form hydronium ions by protonating water. So, this is the compound on our list which produces the most hydronium ions.