# Amount of substance

The section on amount of substance revises, builds upon and extends the work students will have done on the mole concept and chemical calculations at GCSE. Balanced chemical equations are usually given for all but the simplest of reactions. Students will be familiar with many of the types of manipulation in this section such as calculation of empirical and molecular formulae, reacting mass calculations, molar volumes and calculations on solutions, but will very often need further practise and consolidation to become confident.

Students are expected to have a much clearer understanding of the mole as a concept, and resources one and seven are included in order to address this issue. Although they will have met the idea of molar volume at GCSE, this is extended at A level to include the ideal gases equation (pV = nRT).

In addition, problems at A level will frequently require application of ideas from different areas in order to arrive at a solution to one problem. For example, a question may start as a reacting mass type problem, but then require calculation of the concentration of a product gas which is dissolved in a given volume of water. That questions require switching from one tool to another in this way is one of the reasons that students can find this area of the syllabus difficult. It is crucialy important for this reason that students can clearly identify the 'type' of problem that is being posed at each stage of the process so that they are able to select the correct approach from their 'toolkit'.

The resource list draws together a number of activities to provide practise in this area of the syllabus and includes both theory and practical activities designed to place the concepts and skills required in an interesting context.

Whilst this list provides a source of information and ideas for experimental work, it is important to note that recommendations can date very quickly. Do NOT follow suggestions which conflict with current advice from CLEAPSS, SSERC or other recent safety guides. eLibrary users are responsible for ensuring that any activity, including practical work, which they carry out is consistent with current regulations related to Health and Safety and that they carry an appropriate risk assessment. Further information is provided in our Health and Safety guidance.

## Links and Resources

### Unit S1: The Mole *suitable for home teaching*

This resource is part of the ILPAC suite of materials and is a source of ideas and examples related to areas of the section of the A level syllabus on amount of substance. The exercises starting on page six (exercises one to four) are a useful way of approaching the concept of relative atomic mass for students who are having difficulty with the idea of relative quantities. How Avogadro's number was measured is a question frequently asked by students at this level. Experiment one on page 21 guides students through estimating Avogadro's Number experimentally. Answers to the problems in the text are included at the back of the resource.

### Unit P1: The Gaseous State *suitable for home teaching*

This is another comprehensive resource from the ILPAC collection which provides much detailed background material on topics relating to molar volume and the ideal gases equation. The resource would be useful for teachers when planning lessons for this module, but also many of the exercises, with a few dated exceptions, are still useful and pertinent to current syllabuses and can be scanned or retyped and edited to provide class or homework material. This is an invaluable resource for planning, and for setting example problems to support teaching and learning.

### Astounding numbers *suitable for home teaching*

This resource, aimed at gifted and talented students, gets students to think about the mole as a quantity, and puts Avogadro's number in context.

Students and teachers alike will know that Avogadro's number has a value of 6.02 x 10^{23}, but to most, the number has little meaning. Section A of this resource gets students to think about what a mole would be like in terms of familiar contexts. For example by asking "How big is an ocean containing one mole of raindrops". One example, with dramatic impact, is to ask how long it would take to count to Avogadro's number at a rate of one per second, then compare this number in years to the lifetime of the universe (about 1.5 x 10^{10} years). This is not included, but is similar to the calculation in the resource (section A, question one, page two) where the time required is related to Earth lifetimes (the answer is about 1.2 million* universe lifetimes*, yet this number of carbon atoms can be held in the palm of your hand!).

The resource is targeted as an extension activity, but individual activities could be used as class or teacher led resources. Section B includes several activities to give students further general practise on number and estimation, both important skills in this section of the course.

### 1. Quantitative Chemistry *suitable for home teaching*

This resource comprises a pack of activity sheets that cover all aspects of chemical calculations, using a variety of different approaches to keep the interest of students. The activities could be used as in class activities, homework or review material at the end of a module. Answer sheets are included.

### To Find the Formula of Hydrated Copper(II) Sulphate

This is a nice resource to reinforce students' ability to calculate empirical formulae from data, specifically calculating the empirical formula of a hydrated salt. Students may or may not have covered this type of calculation at GCSE but at A level a greater understanding of the method and sources of errors is needed.

Students might be asked, for example, why a lid is not used in this experiment, whilst a similar experiment where we work out the empirical formula of magnesium oxide by heating magnesium ribbon in a crucible would require a lid. The resource does not include heating to constant mass and since this is something the students need to understand this could be added in to the experiment and/or discussed as an extension to the activity.

### The Volume of One Mole of Hydrogen Gas

This resource involves reaction of an accurately measured mass of magnesium ribbon with hydrochloric acid, and subsequent measurement of the volume of hydrogen gas evolved by collection over 'water' in an inverted burette. From the quantity of magnesium reacted, the volume of gas collected, and the stoichiometric equation, students calculate the molar volume (Vm) of a gas under ambient conditions.

Although collection of gases over water in inverted burettes is fairly standard, the methodology itself is somewhat novel in the respect that the burette contains not water, but hydrochloric acid, which as well as being a reactant, allows measurement of the volume of evolved gas by displacement.

Students could be asked to consider the experimental design. For example, "Why is the burette half filled with hydrochloric acid and 'topped' with water? Why does the amount of water added not matter?".

The calculated value for Vm will depend on two factors; 1) accuracy of the measurements made and 2) ambient temperature and pressure. An extension might include a consideration of errors including sources of error and error estimates, and/or correcting the volume to standard conditions (s.t.p and r.t.p).

### Neutralisation of Indigestion Tablets

This is a very simple but nice activity using volumetric analysis (titration) to investigate the efficacy of indigestion tablets. The 'data' mentioned in the question at the end of the resource would need to be established from the tablets purchased for the activity. How the students decide which tablet is 'best' is left open and could be kept simple (for example acid neutralised per gram) or could be extended by taking into account a cost benefit type of analysis.

### Problem 6: Acid Erosion

This is a nice resource which, if used as presented, would be more suitable as an A level project. However, it could easily be adapted in order to provide students with a real context within which to practise the concepts and skills related to acid base titrations, once the basic technique of volumetric analysis has been taught. pKa, titration curves and indicator selection are part of A level, but the idea of strong and weak acids should be familiar to most students at AS level. As such the initial questions included in the resource, if used selectively, can still form an introduction to the activity. Useful summary notes for the teacher, and answer sheets are included.

More able students will notice that there is no carboxyl group in ascorbic acid, and researching why the hilighted proton is acidic would be an interesting extension activity taking students' concepts of acidic groups beyond the confines of the A level syllabus.

The answer is that the negative charge produced on the oxygen is delocalised over the alkene and carbonyl functional groups and therefore stabilised. On ionisation the sp^{3 }hybridised oxygen atom of the alcohol rehybridises to an sp^{2 }state. The negative charge (a lone pair), formally on oxygen, is now in a p orbital which can overlap with the p orbitals of the conjugated alkene and carbonyl groups to form a delocalised pi system analagous to that in benzene for example.

### Estimating the Concentration of Domestic Bleach

This resource combines a demonstration with calculations on molar volume and reacting mass type problems, and could be used to add context and interest to this section of the syllabus. It should be stressed that this method gives an estimate* *and not an accurate measure of concentration. It might be a useful extension exercise to get students to consider why* *this method can only give an estimate of concentration.

### Ice Cores

This resource provides an interestting context in which to introduce quantitative and qualitative analysis. Whilst the whole activity goes beyong the scope of the section on amount of substance, it could very easily be adapted to provide an interesting context in which to carry out volumetric analysis. In this sense it is an alternative to the earlier resource Problem 6 : acid erosion.

Subject(s) | Science |
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Tags | n.a |

Age | 16-19 |

Last updated | 12 July 2016 |

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URL | https://www.stem.org.uk/lxpq9 |