Practical investigations
Read our general notes on Risk Assessment
Planning an investigation to find the relative atomic mass of lithium
We have received several requests asking us about various aspects of this investigation. Please note that if you are following the Nuffield course this investigation is suitable for practice but not for an investigation submitted as A2 coursework. In the Nuffield course the A2 investigation must relate to the content of the A2 topics in the specification.
You need to start by being clear about the way in which you can use measurements of the quantities of reactants and products of a reaction of lithium to arrive at a value for the metal’s relative atomic mass.
The methods to consider need to be based on a reaction (or reactions) of lithium metal which form products which can be measured as accurately as possible.
Lithium’s reaction with water provides a gaseous product and an aqueous solution of an alkali. The volume or mass of the gas formed can be measured and the quantity of alkali formed can be deduced via a titration with an appropriate acid.
You are likely to know something about lithium from your pre-16 courses. Even so you should find out a more about the physical and chemical properties of lithium from reference books, web sites or CD-ROM data bases. Some of these may well have a bearing on the approaches you decide to adopt during this investigation.
These notes concentrate on your planning of the investigation. See also the general tutorial on planning.
Practical details
(1) The first problem to overcome is how to measure the mass of lithium accurately. Lithium is stored under oil to minimise oxidation by the oxygen, water and carbon dioxide in the air. So, before weighing, you will need to cut a piece of the metal under oil, and then find a reliable way of removing the coating of oil from this lump.
Consider using something suitably absorbent and/or a solvent for the oil such as hexane. However, given lithium’s rapid reaction with the air, you need to work quickly. Then you will have to think of a means of weighing the lump as accurately as possible before it oxidises any further.
(2) You will then have to think of a reliable means of collecting all the gas formed from the reaction. If you decide to measure volume, will the use of a gas syringe be best, or will you be using an inverted item of graduated glassware to collect the gas over water? If you think that measuring the loss in mass of the solution due to the evolution of gas is a feasible method, how will you accomplish this? Will the estimated loss in mass for this particular gas be sufficient for your laboratory balance to measure? How much gas do you need to produce as the metal reacts for your measurements to be sufficiently accurate? If measuring volume, how will you use the equation to estimate a suitable mass of lithium to use?
In your planning you should list and review possible approaches and then you should carry out trials to decide finally the best technique.
(3) Next you might want to decide how to measure the amount of alkali, in moles, produced, via a suitable titration. Which acid will you use? How will you prepare a standard solution of this acid? Will its concentration be reliable for use as a standard? Which indicator will you select (Topic 14 of the Nuffield Students’ Book will supply you with some important information about the theory of indicators)? The alkali formed in the reaction of lithium with water reacts quickly with carbon dioxide from the air. What is formed? Will this affect the manner in which you carry out the titration?
(4) You will need to carry out a detailed risk assessment of all your procedures. Remember that lithium and the products of this reaction are potentially hazardous, and that whatever techniques you choose to adopt, 'risks' must be kept to an acceptable minimum. Your teacher should have a supply of the relevant forms which you must fill in, and, of course, you must gain his/her approval before you start any practical work.
(5) Your planning should also include an assessment of likely sources of error.
How might you estimate the uncertainty in the measurements at each stage? Which measurements are critical to the accuracy of your final result?
See also the general tutorial about measurement accuracy and the web guide about experimental error.
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updated: 15 October 2003
