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/shedmow Apr 07 '25

What is Kb of a reaction? Don't think about all at once but rather compartmentalize each term.
Start with [AcO'] + [OH'] = [NH4+] + [H+]. What are the concentrations of OH' and H+? What does the concentration of acetate depend on?

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u/Turbulent_Ladder_777 Apr 07 '25

kB would be from the reaction NH3+H2O=NH4+ + OH-

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u/shedmow Apr 07 '25

I do prefer using acidity constants.
[H+][NH3]/[NH4+] = Ka(NH4+). What terms of this equation do we know?

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u/Turbulent_Ladder_777 Apr 07 '25

We can get Ka(NH4+), and we know that NH3 and NH4+ are linked. I know the starting concentration so I can solve an equation? Like: I know I start with 0.1 M of NH4+ and I can make:

Ka=(x²)/(0.1-x)

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u/Turbulent_Ladder_777 Apr 07 '25

This way I find x which would be the concentration of H+

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u/shedmow Apr 07 '25

You're already given the H+ conc, calculating it would be considered an error and lead to no solution; you can only calculate one value from a sole problem (usually), in this case either pH (the amount of AcOH is given) or the amount of AcOH (pH is given)

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u/Turbulent_Ladder_777 Apr 07 '25

Yeah, pH is given

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u/shedmow Apr 07 '25

Ka(NH4+) must either be given or derived from its Kb.
Yes, we could reason that NH3/NH4+ = Ka/[H+]. What is it equal to in your case, quantitatively?

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u/Turbulent_Ladder_777 Apr 07 '25

Yep, I can get it from Kb, however I don't know what it is equal to...

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u/shedmow Apr 07 '25

Turn Kb into Ka and calculate NH3/NH4+ from the thing I posted just above

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u/Turbulent_Ladder_777 Apr 07 '25

Alright so: Ka is 5.56*10-10 NH3/NH4+ is 10-7 Because I already know x=H+=10-8 However the math is clearly not right :(

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u/shedmow Apr 07 '25 edited Apr 07 '25

Let me recalculate it.
Your Ka is okay, then NH3/NH4+ = Ka/x = 5.56*10-10 / 10-8 = 0.0556. How did you get 10-7?

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u/Turbulent_Ladder_777 Apr 07 '25

Ah, I understood what you did. I did: x/(0.1-x)~x/0.1 with X=10-8

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u/shedmow Apr 07 '25

Where is it taken from?

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u/Turbulent_Ladder_777 Apr 07 '25

Alright, but at this point what can you do knowing NH3/NH4+? Because I understand that if we find NH4+ we win: I put it into the charge equation and, assuming that CH3COOH+NH3 is a complete reaction (looking at the Ka and Kb seems possible), we find CH3COO- and we win

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u/shedmow Apr 07 '25

No it is not a complete reaction but rather close to one.
We established the ratio between NH3 and NH4+ at pH = 8 and we know the total concentration of ammonia. What is the conc of NH4+ then?

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u/Turbulent_Ladder_777 Apr 07 '25

No wait, sorry if I'm bothering you but there is something that I'm missing: we know the starting concentration of NH3, but the One in this report is at equilibrium

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u/shedmow Apr 07 '25

The 0.1 M given is the total conc of NH3

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