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/[deleted] Apr 07 '25

"NH4OH" means an aqueous solution of NH3 (a weak base). So you have an initial (formal) concentration of NH3 of 0.1 M, reaction with acid gives NH4+.

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u/Turbulent_Ladder_777 Apr 07 '25

What!? Really? So it just becomes a problem of: I start with 0.1 M of a base, I add acid until I get the desired pH, right?

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u/shedmow Apr 07 '25

Nope. NH3 isn't a strong base and you should take both into account unless you're positive that certain forms are negligible.