Video Transcript
Which of the following equations
can be used to help calculate the concentration of an acid or a base following a
titration experiment? (A) 𝑛 acid divided by 𝑀 acid
equals 𝑛 base times 𝑉 base divided by 𝑀 base. (B) 𝑛 acid times 𝑉 acid divided
by 𝑀 acid equals 𝑀 base times 𝑉 base divided by 𝑛 base. (C) 𝑀 acid times 𝑛 acid divided
by 𝑉 acid equals 𝑛 base divided by 𝑉 base. (D) 𝑀 acid divided by 𝑉 acid
times 𝑛 acid equals 𝑀 base times 𝑉 base divided by 𝑛 base. (E) 𝑀 acid times 𝑉 acid divided
by 𝑛 acid equals 𝑀 base times 𝑉 base divided by 𝑛 base.
In a titration, a chemist
determines the concentration of an analyte using a known concentration of a second
solution. If we wanted to calculate the
concentration of an acid or base, then we could carry out an acid–base titration
experiment. An accurately known volume of the
analyte is added into a conical flask. An acid–base indicator, such as
phenolphthalein, is also added to the flask. The burette is filled with the
titrant, which is the solution that has a known concentration. Since its concentration has already
been accurately determined, the titrant is referred to as a standard solution.
During the experiment, the standard
solution is slowly added to the conical flask containing the analyte. A neutralization reaction occurs
between the acid and the base as they are mixed together in the flask. Once the indicator changes color,
the endpoint has been reached, which means that all of the analyte in the conical
flask has reacted. At this point, an eye-level reading
of the volume of the standard solution in the burette is made.
Now, let’s make a list of the known
quantities and types of measurements made during the titration experiment. Let’s say that the analyte is the
acid and the standard solution is the base. At the end of the experiment, we
will know the volume of the base that was needed to neutralize the acid. Since the base is a standard
solution, we also already know its molar concentration, or molarity. If we multiplied the volume of the
base in liters by the molarity of the base in moles per liter, we could determine
the amount of moles of base that reacted.
But because we want to determine
the molarity of the acid, we need to make an important conversion. We need to multiply the number of
moles of base by the molar ratio of acid to base, which is represented by the
stoichiometric coefficients in the balanced chemical equation.
We can summarize the calculation
steps that we have completed so far as multiplying the volume of the base by the
molarity of the base and then dividing by the coefficient of the base from the
balanced equation.
Now since we know the number of
moles of base that reacts is related to the number of moles of acid that reacts by
the ratio of the coefficients, we can create a proportion with a similar expression
for the acid. In the example we’ve been
discussing, we would know the volume of acid that we started with in the conical
flask. And we would also know the
coefficient of the acid from the balanced equation. So this equation would allow us to
calculate the unknown concentration of the acid.
In conclusion, the equation that
can be used to help calculate the concentration of an acid or base following a
titration experiment is answer choice (E): 𝑀 acid times 𝑉 acid divided by 𝑛 acid
equals 𝑀 base times 𝑉 base divided by 𝑛 base.
