Chemguide: Support for CIE A level Chemistry

Learning outcome 1.5(c)

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

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

This material is covered in 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:

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. You probably won't be able to do Problem 6 because it needs you to know about the molar volume of a gas.

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 statement 1.5(c), 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 2010 (last modified March 2014)