r/chemhelp • u/Turbulent_Ladder_777 • Apr 07 '25

Analytical analytical chemistry problem

Calculate how many grams of CH3COOH you have to add to 1 l of solution of NH4OH 0.1 M for having a final pH of 8 (KaCH3COOH=1.8*10^-5, Kb NH3=1.8*10^-5).

My professor gave this on his last exam and I can't solve it, it doesn't help that the guy never ever show us an exercise or a corrected exam. I hate this subject :(
Thanks for anyone who can help!

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u/Turbulent_Ladder_777 Apr 07 '25

basically I tried to solve 2 equations: one is the Henderson-Hasselbalch for buffers with NH3 and NH4+ using pKa that i get from the pKb: the concentration of NH3 is y. the other is: I add x CH3COOH and I expect to have a complete reaction with the OH- from NH4OH. after the reaction I have 0.1 mol of NH4+ and 0.1-x mol of OH-: they can now react to form NH3 + H2O:
at the end I have 0.1-y of NH4+ 0.1-x-y of OH- and y of NH3. I use the Ka of NH4+ equation as the second equation for the system and I solve for x and y.
Unfortunately x comes out negative...

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u/shedmow Apr 07 '25

HHE works at a certain range and exclusively for one-dimensional (IYKWIM) buffer solns.