Chemguide: Support for CIE A level Chemistry

Learning outcome 2.4

Reacting masses and volumes (of solutions and gases)

This statement deals with simple calculations from equations.

Before you go on, you should find and read the statements in your copy of the syllabus.

Nearly all you need is included in detail in my A level chemistry calculations book, and so I'm afraid I can't repeat any of it here.

Whether you use my book or an alternative, it is essential that you quickly get confident with these calculations. Spend as long as necessary to make sure that you can do them without problem.

The sad truth is that if you go into the rest of the course feeling that you can't do chemistry sums (and that's all they are - "calculations" makes them sound much too difficult!), then you won't get a good grade.

I am going to give details of which bits of my book you should refer to, but obviously that doesn't mean that you have got to use it. But whatever you do, find a book that suits you. The only way to learn to do chemistry sums is to practise and practise. This is much more easily done from a book than from any other source.

The only real (but less good) alternative is to get hold of every exam paper CIE have set on this syllabus, and work through all the calculations you can find, and check them using the mark schemes and Examiner's Reports.

The very best way, of course, is to use a good book and past exam papers!

Learning outcome 2.4.1(a)

This covers simple calculations from equations involving the masses of substances. In my book, you will find it starts on page 33, and runs to the end of the chapter. Don't forget to look at percentage yield calculations, including at least Example 22 and preferably Example 23 as well.

At the end of the section, do both Problems 21 and 22, and Problems 33 to 36.

Learning outcome 2.4.1(b)

This is covered at the beginning of Chapter 3 of my book. You will need pages 42 to 45.

You will also need to look at simple calculations involving the molar volume of a gas on pages 46 to 48. You will find values for molar volumes at stp and rtp in the Data Booklet during an exam.

The ideal gas equation (starting on page 48) is required, but is discussed in a later section (section 4).

Learning outcome 2.4.1(c)

This takes up the whole of chapter 4 in my book, but there are bits at the end which you probably don't need because they aren't specifically mentioned by the syllabus, and go beyond anything asked so far.

Don't start looking at titration calculations without first working through pages 55 to 59.

Ignore pages 59 to 61 for now - that is relevant to learning outcome 2.4.1(e).

In the titrations section, make sure that you are completely confident about everything up to page 68 before you go any further. Then continue as far as the bottom of page 70. You are unlikely to be asked anything as complicated as Example 17 without a lot of step-by-step guidance, so don't worry about it at this stage. The same is true of back titrations.

Double indicator titrations aren't mentioned at all by the syllabus, and can't be asked.

In the Revision Problems at the end of the chapter, ignore Problems 20 and 22 (because they involve molar volumes of gases). The other questions up to Problem 25 should be straightforward. After that, try some of the later ones if you have the confidence. You won't be able to do Problems 31 and 32 because they involve double indicator titrations.

Learning outcome 2.4.1(d)

This is about limiting reagents and excess reagents. When you do experiments it isn't unusual to have one reagent in excess so that not all of it gets used up. That may be for rate of reaction reasons, or because the other reagent is more expensive and so it is important not to waste any, or various other reasons specific to a particular reaction.

The reagent not in excess is known as the limiting reagent. The yield you get is dependent only on that reagent, and so that is the only one you need to consider when doing a calculation.

It may be that you don't know which is the limiting reagent if you are given amounts of everything in the reaction. In that case, you have to work out the number of moles of everything present and see which you have least of compared with equation proportions.

Suppose the equation says that 1 mole of A reacts with 1 mole of B.

Suppose that when you work out the number of moles of A and B in the mixture, you find that you have 0.241 moles of A and 0.385 moles of B.

Obviously, you have more B than you need, and the yield of product can only depend on the amount of A you have. You can ignore B for the rest of the calculation.

You will find a fairly scary looking example of this on page 37 Example 24 of my book. It isn't actually as scary as it looks!

The comment about significant figures

It is important to give your answers rounded to a proper number of significant figures.

It is essential that you get this right. CIE mark schemes show that they frequently deduct a mark, even if the calculation is otherwise correct, for what they call "misuse of significant figures". One mark doesn't sound a lot but, in the worst case, it could be enough to prevent you getting whatever grade you need for the course you want to do in the future.

The general rule is that you shouldn't quote a final answer to more significant figures than the least accurate number you are working with.

So, for example, suppose you are doing a calculation where you are working with the following numbers: 25 cm3, 23.6 cm3 and 4.51 g.

Look at the number of significant figures each of these numbers is quoted to. The second and third numbers are to 3 significant figures, but the first one is only quoted to 2. That means that your answer shouldn't be quoted to more than 2 either.

However, if the first figure had been given to you as 25.0 cm3 (which is to 3 significant figures), then you should quote your answer to 3 significant figures as well.

Suppose your calculator gave the answer to whatever calculation you are doing as 93.2. That's obviously to 3 significant figures and isn't a problem. However, suppose the answer came off your calculator as exactly 93, but the number of significant figures in the numbers you are working with justify quoting your answer to 3 significant figures.

In that case, you shouldn't give your answer as 93, but as 93.0. By writing the final zero, you show that your answer is actually to 3 significant figures; by leaving it out, it implies it is only to 2.

If an exam question asks for a particular number of significant figures, then obviously that is what you must give, irrespective of anything I might have said above.

Another problem arises if you write down intermediate answers on your way to your final answer. You must be careful not to over-round these, otherwise you will get small errors in your final answer. If you feel more comfortable writing down intermediate answers, it is a good idea to write them down to at least one more significant figure than you will use in your final answer.

In fact, it is much better not to round at all as you go along if at all possible.

The only exception to this comes in a structured question where you obviously have to write down answers to each step individually. In that case, each step should be shown to the correct number of significant figures, and that figure must then be used in the calculation of the next step.

And finally . . .

If in doubt, you will rarely go wrong in a typical exam by quoting your answers to 3 significant figures.

Learning outcome 2.4.1(e)

This statement deals with working out the numbers of moles of things taking part in reactions by doing experiments.

This material is covered on pages 59 - 61 of my A level chemistry calculations book, and so I'm afraid I can't repeat it here, apart from some general comments.

What does "stoichiometry" mean?

This is a big word for a simple idea. It refers to the proportions of things either reacting or combining.

For example, in compounds, it refers to the ratio in which the atoms are combined together. Water has a stoichiometry of 2 hydrogens to 1 oxygen.

In the sense that the word is used in this syllabus statement, it refers to reacting proportions in an equation. For example, when hydrogen and oxygen react to form water:

2H2 + O2     2H2O

The stoichiometry shows that 2 moles of hydrogen react with 1 mole of oxygen to give 2 moles of water.

Finding the stoichiometry of a reaction

In principle, this is quite simple. If you did a reaction between substances A and B, and took suitable measurements, you could find out how many moles of A you started with, and how many moles of B, and then you could easily see what ratio they reacted in.

If you also took some measurements of the products formed, you could also see how this related to the amounts of reactants. This gives you a way of working out the equation.

You will find some examples of this on pages 59 to 61 of my book.

Don't make any attempt to learn the examples in the book. If you are asked a question about this, you will be guided through any calculation, and given plenty of information in the question. As long as you can do mole sums from previous statements, you shouldn't have any problems.

If you can find a copy of the paper, there is a question on this on November 2007 Paper 2 Q3. Unfortunately, this is not available for student download from the CIE website, but your teacher should be able to get a copy from the secure teachers' part of the site.

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© Jim Clark 2019