Which graph could represent the pH
of a 0.05 molar solution of hydrochloric acid versus the volume of a 0.05 molar
solution of potassium hydroxide that is added to it.
We’re looking to construct a graph
of the pH of an acidic solution versus the volume of potassium hydroxide, which is
the base that’s added to it. This means that the pH should go on
the 𝑦-axis and the volume of potassium hydroxide that we’re adding should go on the
𝑥-axis. In the experiment that’s described
in the question, we’re adding a base to an acid and monitoring the pH. So, this sounds like a titration
experiment. So, now let’s think about how the
pH of our solution will change as we slowly titrate it with more base.
Remember that the pH is defined as
the negative log of the concentration of hydronium ions in the solution. A low pH means that we have a high
concentration of hydronium ions in the solution, which means that the solution will
be acidic. If the pH is high, that means the
concentration of hydronium ions will be low, which means our solution will be
basic. In a basic solution, we have a high
concentration of hydroxide, or OH⁻ ions.
Let’s think about what the pH of
our solution will be before we add any base. Since we’re starting off with a
solution of hydrochloric acid, we’d expect the pH to be low because the solution is
acidic. As we begin to add base, it will
start to react with the acid and neutralize it, which will form water and a salt, in
this case KCl, which is called potassium chloride. Once we start to add the base to
our solution, the pH will begin to increase because the base is reacting with the
acid, which means that the concentration of our acid and, therefore, the
concentration of the hydronium ions in the solution are going down.
The increase in the pH will be
quite slow at first because the base is being totally used up to react with the
acid. The pH of our solution will begin
to increase much more quickly once we get close to the equivalence point, which is
where the moles of hydroxide from the base that we’ve added are equal to the moles
of hydronium that were present in the original acidic solution.
Since the products of this reaction
are water and a salt, which are both neutral, we’d expect the equivalence point for
this reaction to occur at a pH seven. After the equivalence point, all of
the acid has been used up, but we’re still adding base to the solution. This means that the pH will
continue to increase since we’re increasing the concentration of hydroxide ions in
the solution. Now, that we’ve walked through the
whole titration experiment, we can connect the dots to finish our graph.
You should know that in this
question, we were working with a monoprotic acid, which is an acid that only has one
hydrogen to react with the base. If we had a diprotic acid, which is
an acid that has two hydrogens, the resulting titration curve would look much
different. Either way, our graph most closely
resembles answer choice E, which is the correct graph to represent the pH of a
hydrochloric acid solution as we add potassium hydroxide to it.