[AntonioVerde]

How should I solve an exercise like that? (like with NH3OH and CH3COOH)
I searched online and i found something like that, is it right?

"Weak acid + weak base: Find the limiting reagent. If the weak acid is the limiting reagent, calculate the moles of excess weak base and determine the concentration of [OH-] from the excess weak base. If the weak base is the limiting reagent, calculate the moles of excess weak acid and determine the concentration of [H+] from the excess weak acid."

[Borek]

Not a trivial situation, and no, calculating just from the excess weak acid/base is not a way to go (can be sometimes, but is not guaranteed to give a correct answer).

compare

https://www.chembuddy.com/calculation-of-pH-of-salt-solutions

and

https://www.chembuddy.com/simplified-method-of-calculation-of-pH-of-a-salt-solution

[AntonioVerde]

Not a trivial situation, and no, calculating just from the excess weak acid/base is not a way to go (can be sometimes, but is not guaranteed to give a correct answer).

compare

https://www.chembuddy.com/calculation-of-pH-of-salt-solutions

and

https://www.chembuddy.com/simplified-method-of-calculation-of-pH-of-a-salt-solution

Thanks for your help and time but it doesn't really give me an answer to how to resolve an exercise like that:
"Calculate the pH of a solution containing 3.3×10-1 g di NH4OH (Kb = 1.8×10-5) in 750 mL of water. Calculate the pH of the solution when 28 mg di CH3COOH dissolved in 500 mL of H2O are added."

[Borek]

I am afraid there is no simple answer - these are difficult problems, that require more general approach.

You can try to solve such a problem using Henderson-Hasselbalch equation (treat the solution as a buffer and assume neutralization was close completion). Sadly, without doing some checks you will never know if the answer is OK.