Calculate the pH of a 0.2 M solution of NH4CH3COO. Use activity and charge balan

Question

Calculate the pH of a 0.2 M solution of NH4CH3COO. Use activity and charge balance to do so. (my professor gave us a hint

and said it would be near neutral but i keep getting a ph around 5 so

please show all your work so I can see what I am doing wrong and thank you in advance)

Calculate the pH of a 0.2 M solution of NH4CH3COO. Use activity and charge balance to do so. (my professor gave us a hint

and said it would be near neutral but i keep getting a ph around 5 so

please show all your work so I can see what I am doing wrong and thank you in advance)

Explanation / Answer

CH3COO- + H2O <--------------> CH3COOH + OH-
kb=kw /ka= 5.6 * 10-10
NH4+ + H2O <--------------> NH4OH + H+
ka = 5.6 * 10-10
To determine [OH-]
kb = [OH][CH3COOH]/ [CH3COO-]

let x = [OH] = [CH3COOH]

[CH3COO-] [CH3COOH] [OH]

I 0.2 0 0

C -x +x +x

E 0.2-x x x

[CH3COO-] = 0.20 - x ~ (approximated) to 0.20
Hence,
kb = x2/0.20

x = [OH-] = sqrt (0.20 x kb)

If you do the same thing with the second hydrolysis

x = [H+] = sqrt (0.20 x ka)
But ka=kb, so you got equimolar amount of OH- and H+, which will neutralize each other producing a solution with pH = 7.00.
Meaaning, whatever OH- was produced, was neutralized by H+ that was also produced, so you got pH = 7.00
HENCE, CH3COONH4 is a NEUTRAL SALT

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