Which of the following represent the minimum data needed to find the concentration of a solution of an acid by titration with a solution of a base, assuming that the titration is taken to the end point and that the reaction equation is known? I) Concentration of the base. II) Volume of the acid. III) Volume of the base. A) I only. B) II only. C) I and III only. D) II and III only. Or E) I, II, and III.
The question mentions a specific technique, a titration. A burette is filled with a standard solution, usually of the base like sodium hydroxide. And a conical, or Erlenmeyer flask, is filled with a solution of acid. The solution of base is added to the acid until the base neutralizes the acid. We achieve perfect neutralization at the end point. A pH indicator is sometimes used to show when the end point has been reached.
So, what pieces of information do we need to find the concentration of that solution of acid at the bottom? The final value, the concentration of acid, would probably be determined in moles per litre. We calculate a concentration by taking the amount of substance in moles and dividing by the volume of solution. So, along the way, we’re definitely going to need to know what the volume of the acid solution is. That’s II. So, whatever answer we end up with has to include II.
So, how do we work out the amount of acid in our solution? The easiest way of working out the amount of acid we had to start with is by referring to the amount of base needed to neutralize it. To do that, we need the reaction equation. Let’s look at the example of sulfuric acid and sodium hydroxide producing sodium sulfate and water. If we already have the reaction equation, we’ll know that the ratio of acid to base is one to two.
So, the starting amount of acid in moles is equal to the amount of base required to neutralize it multiplied by one divided by two. This is because, in this particular case, we need half as many moles of sulfuric acid to neutralize sodium hydroxide. So, we’ve got as far as working out that in order to work out the concentration of acid, we need the amount of acid. And in order to work out the amount of acid, we need to know the amount of base required to neutralize the acid.
But amount of base doesn’t appear in our statements. The only things we have left are the concentration of the basic solution and the volume of the basic solution. Well, we can calculate the amount of base in moles required to neutralize our acid by taking the concentration of base and multiplying by its volume. So, we’ve just demonstrated we need to know the concentration of base. And we need to know the volume of the base. When we know the value for these, we can work out the amount of base.
We can combine this information with the balanced chemical equation to work out the amount of acid. And we can combine this with the volume of acid to work out our initial concentration of acid. In practice, the way this would usually go is we’d start off with a solution of base where we know precisely the concentration. And we start off with a solution of acid where we don’t know the concentration, but we do know the volume. We’d add the basic solution to the acid until we reach the end point, and measure the difference in volume on the burette.
Then, we can combine all these bits of information to calculate the unknown concentration of acid. So, we’ve just shown that the minimum data set we need in order to find the concentration of a solution of an acid by titration with a solution of a base, assuming that the titration has taken to the end point and that the reaction equation is known, is the concentration of the base, the volume of the acid, and the volume of the base. Which corresponds to E, 1, 2, and 3. If we lack any of these pieces of information, we cannot complete the calculation.