Practical investigations
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I am doing my chemistry coursework where I have to calculate percentage errors. I understand how to calculate them but I don’t know the percentage errors for all my equipment. I was hoping that you could tell me the error using a 1000 cm3 volumetric flask and a 5 cm3 pipette.
Igloo writes …
The volume to which any piece of equipment can be read depends on:
[1] the stated accuracy of the equipment itself (e.g. is it a grade “A” flask or a less reliable and cheaper one?). Is the temperature of the liquid being measured the same as the stamped value on the glassware?
[2] how “accurate” the worker is, i.e. is he/she very careful and precise in his/her work or does he/she tend towards clumsiness?
The LGC (formerly the Laboratory of the Government Chemist) publishes a useful guide to analytical chemistry which includes a table of measurement uncertainties of typical laboratory equipment. The uncertainty values they quote include:
Here are some of the uncertainties:
Note that your teacher will be much more concerned about whether you can convert your error into a percentage error, whether or not you know how to deal with and combine several errors and whether he/she believes that you have considered all the possible ways in which your results could be adversely affected.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 14 August 2007
