Which of the following substances would have the lowest hydroxide ion concentration when preparing a 0.75 molar solution of it?
What this question is asking is, imagine we make a solution of each of these substances where water is the solvent and the concentration for each of these solutions is 0.75 molar. So we have 0.75 molar HCl and a solution of [0.75] molar KOH and so on. What we need to figure out is which substance would cause the solution to have the lowest hydroxide ion concentration.
The concentration of hydroxide ions in an aqueous solution is tied to the pH. In any sample of water or a aqueous solution, molecules of water are continually reacting with one another to produce hydronium ions and hydroxide ions. We determine the pH by taking the negative log to the base 10 of the hydronium concentration. You might see a hydrogen ion in place of the hydronium ion. These are essentially equivalent. The equilibrium constant for this process is known as the autoionization constant of water. And it’s equal to the hydronium ion concentration multiplied by the hydroxide ion concentration.
So let’s imagine if we add a chemical to the solution that releases hydrogen ions. The concentration of hydronium will go up, the pH will go down, and the concentration of hydroxide ions will also go down. This ties back to the equilibrium constant. If we increase the concentration of hydronium, we have to decrease the concentration of hydroxide to get the same constant value. If we do the reverse where we decrease the hydronium ion concentration, perhaps by adding a base, the pH will rise and so will the concentration of hydroxide. Now, we can look at each of the substances and consider what’s likely to happen when we make a solution of it.
HCl is hydrochloric acid. Hydrochloric acid is an example of a strong acid. So it will produce a solution with a low pH. As we’ve demonstrated, this will also produce a low hydroxide ion concentration, which is what we’re aiming for. Hydrochloric acid is a strong acid, so it dissociates completely in water. Every single molecule of HCl contributes to the acidity. This makes HCl a very good candidate for our answer. We’re going to have a very low hydroxide ion concentration in our 0.75 molar solution of HCl.
Our next candidate is potassium hydroxide, which is a strong base. It’s an Arrhenius base, which means it actually contains the hydroxide ion. So it’s going to contribute to a high hydroxide ion concentration in our solution. So potassium hydroxide is definitely not a correct answer.
The next candidate, potassium bromide, is a neutral salt. Neither potassium ions or bromide irons react significantly with acids or basis. So the pH is not going to differ from that of pure water. So for our solution of potassium bromide, we’d have the normal level of hydroxide ions, as we’d get at about pH seven. But although this is lower than what we’d get for our potassium hydroxide solution, it’s not as low as the value we’d get for our hydrochloric acid solution. So potassium bromide cannot be a correct answer either.
The next candidate is ammonium chloride, which is a weak acid. A weak acid dissociates partially in water, contributing some but not all of the protons to the water. For the case of ammonium chloride, we have the weakly acidic ammonium cation. This reacts with water, producing ammonia and the hydronium ion. But the reaction is reversible, meaning that an equilibrium is established and not all of the ammonium ions disassociate. So this weak acid is going to lower the pH. And we are going to have a lower hydroxide ion concentration. But being a weak acid, ammonium chloride is not going to lower the pH as much as hydrochloric acid will. Meaning, we have one more candidate to look at.
NH₃ is the chemical formula for ammonia, which is a weak base. Adding ammonia to our water will raise the pH and increase the concentration of hydroxide ions. Although it won’t do it to the same degree as the strong base potassium hydroxide. It doesn’t matter much because it’s still moving in the wrong direction. We want the lowest hydroxide ion concentration, which is what we’d achieve with our solution of hydrochloric acid.