Practical investigations
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I'm doing a back titration for aspirin and have just completed it by hydrolysing the aspirin with excess sodium hydroxide and then titrating it with hydrochloric acid using phenolphthalein as the indicator. I am confused as to whether the aspirin reacts with 3 moles of sodium hydroxide or 2 moles and how to work out the mass of aspirin in the sample.
Igloo replies
You are quite correct. There is much confusion over the “correct” equation for the alkaline hydrolysis of aspirin. With excess alkali, three moles of sodium hydroxide are required to destroy one mole of aspirin, but the theory behind the chemistry of the back titration and the subsequent calculation is rather tricky. Fortunately, we can help by referring you to Aspirin investigations (revised February 2007) a tutorial on this website.
About half-way down the tutorial you will find all the information you need, together with the stages needed to carry out your calculations. It’s all a little tricky, so read it all through very slowly and carefully, and hopefully everything should become clearer.
Please remember to include the chemistry-react website in your acknowledgements at the end of your write-up.
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: 06 April 2005
