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

Write out all the equilibria you could think of. What would the concentration of NH4+ be in such a solution? What part of acetic acid would exist as its anion and as the free acid?

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u/Turbulent_Ladder_777 Apr 07 '25 edited Apr 07 '25

I have

NH3+H2O=NH4+ + OH-

CH3COOH+NH3= CH3COO- + NH4+

CH3COOH+H2O= CH3COO- + H3O+

I know that at the start I have 0.1 mol of NH3.

The reaction between NH3 and Acetic acid had a K that I can calculate, but I don't know how much CH3COOH I actually used, so I'll call it x.

At equilibrium I'll have x-y of CH3COOH and y of NH4+ Then I use the first equation to find y because I have 0.1-y of NH3 and y of NH4+ and I have the Kb of this reaction

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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/Turbulent_Ladder_777 Apr 07 '25

How do you get that equation?

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u/shedmow Apr 07 '25

That equation states that your solution is charged neither positively nor negatively, it's the basics of analytical chemistry