Titration calculations

Check that you understand the basics of titration calculations.

Sometimes more complicated titration calculations are presented for assessment, but they still require the same techniques. You are usually guided through them step by step. If you are not sure how to do the calculation, always have a guess using the data presented - work out any numbers of moles that you can. Transferred errors can come to your rescue, so don't worry about errors early in the calculation - just keep going. However, if your answer is clearly wrong (eg 2000 M) try to locate the error.

These are the basic skills that you will need:

1 Choosing data

This doesn't usually cause any problems. Ignore any anomalous result (that is any result that differs from the average by more than 0.1 cm³). Average the remaining results.

2 Writing equations

Make sure you are familiar with the topic being covered and that you can write down the key chemical equations involved. You may have access to a textbook, so this may not be a problem, but it's still good practice. You should be able to write down any acid-alkali equation. The iodine-thiosulfate and iron(II)-manganate(VII) reactions are also popular:

2Na₂S₂O₃(aq) + I₂(aq) ⇒ Na₂S₄O₆(aq) + 2NaI(aq)

MnO₄^-(aq) + 8H⁺(aq) + 5Fe²⁺(aq) ⇒Mn²⁺(aq) + 5Fe³⁺(aq) + 4H₂O(l)

3 Working out number of moles

This is the essential part of any titration calculation. You will need to use the equation:

Number of moles (in mol) = concentration (in mol dm^-3) × volume (in dm³)

Also, don't forget that any volumes in cm³ need to be first converted to dm³ by dividing by 1000. If you know the concentration and the volume of a reagent, always work out the number of moles. Make sure you label your answer "Number of moles of 'reagent X' = ". Typically at the end of a calculation, we shall have a number of moles and a volume and need to calculate the concentration. So make sure you can re-arrange the above equation to give concentration.

4 Using equations

This is an essential part of any titration. You may have to do it more than once if there is more than one equation involved. Having worked out the number of moles of one of the substances (using the last equation), we use a chemical equation to see how it is linked to one of the other substance. For example, using the following equation:

IO₃^-(aq) + 5I^-(aq) + 6H⁺(aq) ⇒ 3I₂(aq) + 3H₂O(l)

If I have just calculated the number of moles of iodate ions (IO₃^-), the equation tells me that there are five times as many iodide ions (I^-). We can consider the link between any pair of substances in the equation. So, similarly, if I know the number of moles of hydrogen ions (H⁺), I know there are half as many iodine molecules (I₂).

5 Calculating masses

You should be able to use the following equation to convert number of moles into mass:

Mass of substance (in g) = number of moles (in mol) × molar mass (in g mol^-1)

so, for example, 0.00327 mol of iodine, I₂, weighs 0.00327 × 254 = 0.83 g

6 Calculating percentages

This is usually linked to questions in which we are trying to calculate the percentage purity of a substance. We know the mass of the impure substance, and we have just used a titration calculation to work out the actual mass of a particular chemical in the impure substance. The percentage purity cannot be more than 100%, so the following equation gives the percentage purity:

Check that your answer looks reasonable. If we are checking the purity of a substance, we would expect it to be mostly that substance. A purity of less than 50% is very unlikely.

Exercises

Exercise 1 and exercise 2 are acid-alkali titration calculations (also problem 1 and problem 2).

Exercise 3 and exercise 4 are iodine-thiosulfate titration calculations.

Exercise 5 and exercise 6 are iron(II)-manganate(VII) titration calculations.

There are more examples in the section on back titrations.