Hi, I'm doing my Aspirin Coursework at the moment and I'm a little confused on the calculations of back titrations. I've tried it following the method here:

http://www.chemistry-react.org/go/Tutorial/Tutorial_21681.html but I'm not sure I used the correct numbers in the right place!

Using 0.5g of an aspirin sample I made, I added 4.375cm

^{3} of 0.5moldm

^{-3} NaOH until the phenolphthalein indicator remained pink. Then I added a further 14.375cm

^{3} of NaOH in excess. (Therefore my total volume of NaOH added was 18.75cm

^{3})

Then I backtitrated with 10.5cm

^{3} of 0.5moldm

^{-3} HCl to neutralise the excess base.

I need to find out how much ASA is present in the sample, and hence work out the purity of my sample.

Moles HCl required: 0.0105 x 0.5 = 0.00525 mol

Moles NaOH reacting with HCl: 0.00525mol

Moles NaOH added to hydrolyse aspirin: 0.014375 x 0.5 = 0.0071875 mol

Net moles of NaOH reacting with ASA alone: 0.0071875 - 0.00525 = 0.0019375 mol

Since 1 mole ASA reacts with 2 moles NaOH: 0.0019375/2 = 0.00096875 mol

Therefore, mass of ASA in sample: 0.00096875 x 180 = 0.174375g

Hence % purity: (0.17435/0.5) x 100 = 34.87%

Would this be correct?

Thank you for any help you can give me!