[agrocadabra]

Ok so I have a lab on determing the mass of asprin (acetylsalicyclic acid) in a tablet. The tablet is dissolved in a mixture of de-ionised water and ethanol and the asprin in this solution is titrated with 0.1020M NaOH using phenolphthalein indicator. It was found that 16.40mL of the NaOH was required for complete reaction with the asprin. Calculate the moles and then the mass in milligrams of asprin in the tablet:

My attempt:

ok so # of moles if NaOH used: n =cv = 0.1020 * 0.01640 = 0.0016728 moles of NaOH

Now for a complete reaction (this is where I am getting confused) the NaOH will react with the acetylsalicylic acid to produce water right? So my working was:

complete reaction means that we need to use all the H atoms from the acetylsalicylic acid to produce water with the OH molecules. So in asprin we have 9C 8H and 4O, so we require 8NaOH molecules to react with every asprin molecule. Hence amount of asprin for complete reaction (in moles) is: 0.0016728 / 8 = 0.0002091 moles of asprin --> 37.67mg

Anyway please give any feedback and help you are able to. Regards, Agrocadabra

[Borek]

No, most of the hydrogens in the aspirin molecule are not acidic, check the structural formula of the compound:

http://en.wikipedia.org/wiki/Aspirin

[agrocadabra]

ahh beautiful.. so i get 301.362mg of asprin. I just don't know why all of them arent acidic?

[Borek]

Depends on how tightly the hydrogen is bonded with the rest of molecule. In general, hydrogens attached to highly electronegative atoms/groups are much more acidic than the other ones.

[agrocadabra]

Thankyou very much for the insight