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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: le_beda on June 28, 2012, 01:11:24 AM

Title: Ammonium Acetate + OH-, {titration}
Post by: le_beda on June 28, 2012, 01:11:24 AM

Hi everyone,

I am trying to find the pH of 0.04M Ammonium Acetate solution (V=1.5ml) after adding 5*10^-7 moles of NaOH.

pH of the 40mM Ammonium Acetate solution before addition of NaOH was 7. I also found concentrations before addition of NaOH: NH3 = CH3COOH = 0.000221M, and concentration of NH4+=CH3COO-= 0.04M - 0.000221M = 0.039779M.

So if someone can explain how to solve problem and understand what is going on I will really appreciated.

Thank u.

Title: Re: Ammonium Acetate + OH-,
Post by: Borek on June 28, 2012, 04:50:39 AM

I can't think of any simple method at the moment. We are very close to the neutral solution, so obvious approximations (like ignoring H⁺ or OH^- in charge balance) don't hold. Calculation of pH of a salt of a weak acid and a weak base is nicely symmetrical, as because of the charge balance hydrolysis of both conjugates must be almost identical - but here this symmetry is broken. Systematic approach yields a 4^th degree equation (see http://www.chembuddy.com/?left=pH-calculation&right=pH-salt-solution - it is enough to add concentration of Na⁺ to the right side of the equation 11.13 (or 11.15) to get an equation describing the system - but solving it is a PITA.

It is not difficult to get an answer using pH calculator (the one built in Buffer Maker), but whether it suits you depends on what you really need.

Title: Re: Ammonium Acetate + OH-,
Post by: le_beda on June 28, 2012, 09:51:41 AM

I was trying to solve this problem using system of equilibrium equations how we did it in college:

NH4⁺  ::equil:: NH3 + H⁺
CH3COO^-  ::equil:: CH3COOH + OH^-
2H2O  ::equil:: OH^- + H3O⁺
NH4⁺ +CH3COO^-  ::equil:: CH3COOH + NH3

These equations help me to find pH of ammonium acetate (pH=7), and concentation of NH3 and CH3COOH when the system in neutral (0.000221M).

But I do not understand how these equations will change if I add NaOH in there, because OH- will interact with both species NH4+ and CH3COO-...

Thank u for your help.

Title: Re: Ammonium Acetate + OH-,
Post by: AWK on June 28, 2012, 10:14:38 AM

What will happen after addition of this amount of NaOH to ammonium acetate solution?
Has the change of ammonia concentration is relevant to change of pH or not?

Title: Re: Ammonium Acetate + OH-,
Post by: le_beda on June 28, 2012, 10:27:23 AM

What will happen after addition of this amount of NaOH to ammonium acetate solution?The pH will increase from 7.08 to 7.49.


Because I actually did experiment and  measure pH, I definitely know that when I added 5*10^-7 moles of NaOH to 1.5 ml of 40mM ammonium acetate solution, the pH of solution increased from 7.08 to 7.49.

Title: Re: Ammonium Acetate + OH-,
Post by: Borek on June 28, 2012, 10:54:50 AM

Calculations with full model show that pH should change from 7.00 to 7.36 - which most likely means your pH meter is off by 0.08 unit (but the difference of 0.36 is reproduced perfectly).

Note: equation that I pointed you to was derived starting from the equations you listed, plus mass and charge balance. You will need to include them if you want to solve the problem. Basically you are trying to pry open doors when I gave you the key, it is your unquestionable right to do so  :P

Also note, that of the four equations you listed only three are independent.