Practical investigations
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I am currently writing up my A2 chemistry investigation (different methods of analysing aspirin), but am really stuck with the back titration calculations. Is there anywhere I can find similar calculations to follow?
Igloo writes
Use the React website and the search facility to find tutorials on carrying out various types of chemistry calculation; you will also find weblinks to references guiding you through titration calculations.
In the case of your investigation I can summarise the stages for you:
[1] From your back titration reading, and knowing the concentration of acid used, first calculate the moles of sodium hydroxide remaining after the reaction with aspirin.
[2] Calculate the moles of sodium hydroxide added at the start, and subtract from this the moles of sodium hydroxide remaining (part [1]). This gives you the number of moles of sodium hydroxide destroyed by the aspirin.
[3] Knowing the equation for the reaction between aspirin and sodium hydroxide, deduce the moles of aspirin present in the tablets.
[4] Convert the moles of aspirin into a mass in grams, using Mr[aspirin]
[5] Compare this mass with the mass of tablets originally weighed out, and from this you can easily deduce the percentage of aspirin in the tablets, which I presume is what you are trying to determine.
Note about deducing the percentage of aspirin
First convert the number of moles of aspirin you have calculated into a mass in grams, using the relationship:
mass of aspirin in grams = moles of aspirin x molar mass of aspirin
Next, divide this mass by the mass of tablet(s) originally used, and multiply by 100:
Percentage, by mass, of aspirin in tablet(s) = [mass of aspirin / mass of tablet(s)] x 100
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: 04 April 2005
