[Boxxxed]

The pKa fo acetylsalicylic acid, CH3COOC6H4COOH is 4.56.  Calculate the pH at 25oC of an aqueous solution containing 325 mg of aspirin per 100 ml.

How do I know how many moles of acid there are when I don't know the total volume? Do I assume the volume is 100 ml? In which case the moles of acid would be

0.325 grams/168.08gMol = 0.0019 mol
?

[Boxxxed]

Concentration would be 0.019mol/L?

I just realized concentration is what's important here

[Boxxxed]

I did the question and got a ph of 3.15 which is the right answer. However I used the quadratic equation instead of the method of approximations in my notes.

Can someone explain what they did with...

H₃O⁺ = Sqrroot of (0.018)(2.75x10^-5)

= 7.04x10^-4 = x = the change in concentrations

What do they mean by successions as well? They remove the x from denominator because it is negligible but isn't it enough to just solve that equation?

[Borek]

http://www.chembuddy.com/?left=pH-calculation&right=pH-weak-acid-base

It is common to use simplified method if it gives correct result. Nothing wrong with using more complicated one.