Practical investigations
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I'm currently carrying out my back titration method for determining the mass of acetylsalicylic acid in aspirin. I've calculated 40.6 cm³ of NaOH reacted with the acetylsalicylic acid, which means that there were 4.06 M of NaOH that reacted (the concentration was 0.1 M). This means that there were 2.03 M of acetylsalicylic acid in my aspirin tablet (of which I used 0.3 g of). But the gram formula mass of ASA is 180 g, does this not mean that there was 360 g of ASA in 0.3 g of my aspirin tablet? 260308
Corrie writes .....
You are being careless with your use of units. M means mol dm-3, so it is wrong to say you have 4.06 M of NaOH. What you mean is that the amount of NaOH used
= Volume (in dm3) x Conc. (in mol dm-3)
= (40.6/1000) x 0.1
= 4.06 x 10-3 mol
This gives you a more reasonable answer, 103 times smaller - but unfortunately still larger than the mass of your tablet! How accurate was the NaOH concentration? NaOH solutions do tend to 'go off ' on standing by reaction with CO2 in the air, so need to be standardised against acid of known concentration before use in accurate work.
Risk assessment
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updated: 27 March 2008
