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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: xshadow on November 19, 2016, 12:45:13 PM

Title: pH solution of NH3 and CH3COOH
Post by: xshadow on November 19, 2016, 12:45:13 PM
HI :)

How can I calculate the pH of a solution that i get mixing :

1000ml  NH3 10-2 M
1000ml CH3COOH  10-3 M

Now I have this acid-base reaction:

NH3 + CH3COOH ----> NH4CH3COO

But NH4CH3COOH is an ampholytes because in H2O gives two reaction:

1)  CH3COO- + H2O ---> CH3COOH + OH-
2)NH4+  + H2O ---> NH3 + H3O+


So in the reaction environment I should have:

a) [NH4CH3COOH] = 10-3M
b) some remaining NH3 (it was in excess) (I think 10^-2  - 10^-3)


How can I calculate the pH of this solution?
Thanks


Title: Re: pH solution of NH3 and CH3COOH
Post by: AWK on November 19, 2016, 01:50:51 PM
This is a buffer solution.
Title: Re: pH solution of NH3 and CH3COOH
Post by: xshadow on November 19, 2016, 02:31:17 PM
Quote from: AWK on November 19, 2016, 01:50:51 PM
This is a buffer solution.

mmhhh yes...

I should have NH4+/NH3 ...but I have also CH3COO- that I think it gives the reaction:

CH3COO- + H2O ---> CH3COOH + H3O+


SO how can I calculate pH fot this "solutuion" ?
(buffer solution of NH4+/NH3 but also some CH3COO- (coming from the salt) which gives hydrolysis- coming from the salt)

Thanks :)
Title: Re: pH solution of NH3 and CH3COOH
Post by: AWK on November 19, 2016, 03:12:06 PM
Buffer solution always contains mixture od two compounds; salt and weak acid (or base).
Two wise men prepared a special formula for such calculations.
Title: Re: pH solution of NH3 and CH3COOH
Post by: Borek on November 19, 2016, 05:04:45 PM
Assume neutralization went to completion.
Title: Re: pH solution of NH3 and CH3COOH
Post by: xshadow on November 20, 2016, 05:32:34 AM
Quote from: AWK on November 19, 2016, 03:12:06 PM
Buffer solution always contains mixture od two compounds; salt and weak acid (or base).
Two wise men prepared a special formula for such calculations.

yes...

But usually is:

Salt :  NH4Cl
weak base : NH3


So I have a buffer solution of NH4+/NH3  and a strong base Cl- that doesn't hydrolize
BUT I have,in this case , a buffer solution of a chemical compound "A" (NH3/NH4+) and a weak base  CH3COO- that reacts with water (hydrolize)


So how can I calculate pH considering also the protonation of CH3COO- in my buffer solution?
Thanks!
Title: Re: pH solution of NH3 and CH3COOH
Post by: xshadow on November 20, 2016, 05:37:16 AM
Quote from: xshadow on November 20, 2016, 05:32:34 AM
Quote from: AWK on November 19, 2016, 03:12:06 PM
Buffer solution always contains mixture od two compounds; salt and weak acid (or base).
Two wise men prepared a special formula for such calculations.

yes...

But usually is:

Salt :  NH4Cl
weak base : NH3


So I have a buffer solution of NH4+/NH3  and a strong base Cl- that doesn't hydrolize
BUT I have,in this case , a buffer solution of a chemical compound "A" (NH3/NH4+) and a weak base  CH3COO- that reacts with water (hydrolize)


So how can I calculate pH considering also the protonation of CH3COO- in my buffer solution?
Thanks!

I can assume this because I have a buffer solution and a WEAK base CH3COO-??
So I can neglect  the  hydrolisis of CH3COO- in order to give acetic acid?


BUT if I have a LOT of  CH3COO- (much salt) isn't there a formula in order to calculate the pH of this system?
(I usually use formulas obtained with mass and charge balance...)

Thanks!
Title: Re: pH solution of NH3 and CH3COOH
Post by: AWK on November 20, 2016, 06:00:17 AM
For buffer solution a working ratio Bronsted acid/conjugated Bronsted base is from 1/10 to 10/1.
Your data fails in this range. So you can use the most approximated formula for calculation pH of solution (after correct stoichiometry of neutralization - your first post showed the correct way of this stage).
You should write textbook chapter concerning buffer solution. All these informations you can find there.
Title: Re: pH solution of NH3 and CH3COOH
Post by: magician4 on November 20, 2016, 08:23:02 AM
@ AWK:
Quote
For buffer solution a working ratio Bronsted acid/conjugated Bronsted base is from 1/10 to 10/1.
Your data fails in this range.
assuming that with the excess of ammonia given,  the acetic acid becomes more or less completely deprotonized, the resulting ratio of NH3/NH4+ should be like ~ (10-2 - 10-3) / 10-3 = 9 : 1 , which is well within named range


@ xshadow:
you could approximate the situation to be an ammonia / ammonium-buffer of above named composition, and use the calculation belonging to
for higher precision, I'd recommend to use a simplified charge balance - but this might be slightly overdoing here


regards

Ingo
Title: Re: pH solution of NH3 and CH3COOH
Post by: Borek on November 20, 2016, 09:23:49 AM
Quote from: magician4 on November 20, 2016, 08:23:02 AM
@ AWK:
Quote
For buffer solution a working ratio Bronsted acid/conjugated Bronsted base is from 1/10 to 10/1.
Your data fails in this range.
assuming that with the excess of ammonia given,  the acetic acid becomes more or less completely deprotonized, the resulting ratio of NH3/NH4+ should be like ~ (10-2 - 10-3) / 10-3 = 9 : 1 , which is well within named range

I think AWK meant the ratio FALLS in the range and you are both saying exactly the same.
Title: Re: pH solution of NH3 and CH3COOH
Post by: Stronzio2 on May 26, 2018, 05:50:23 AM
K for this reaction = 31578      = ka(ch3cooh)/ ka(nh4)
So
Nh3= 0,0045 M
Ch3cooh= 0,0005 M
Nh4+ = 0,0005 M
Ch3cooh = 0   ( k is quantitative)

H+ = ka (nh4) * 0,0005 / 0.0045 

pH=11-0.80=10.20