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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: duma on October 22, 2014, 02:06:37 AM

Title: not understand how my book said pka=ph when its not even at 1/2 equivalence pt
Post by: duma on October 22, 2014, 02:06:37 AM
What is the predominant form of ammonia in a solution at pH 7.0? Approximately
what fraction is in this form?

􏰀SOLUTION In Appendix B, we find pKa 􏰆 9.24 for the ammonium ion (NH 4,
the conjugate acid of ammonia, NH3). At pH 􏰆 9.24, [NH􏰀4 ] 􏰆 [NH3]. Below pH 9.24, NH􏰀4 will be the predominant form. Because pH 􏰆 7.0 is about 2 pH units below pKa, the quotient [NH3]/[NH􏰀4 ] will be about 1:100. Approximately 99% is in the form NH􏰀4 .

so the book said that the pka is 9.24 for the ammonium ion NH4. Then it said ph = 9.24.

Are they saying pka = ph because they are assuming the ion is at 1/2 equivalence point? I'm confused. How did they get pka=ph? Is it a typo? We're not even talking about titrations in this example.

Also I don't understand how they got 1:100 due ph of 7 being 2 units lower then the ph of 9.24. How does that work?
Title: Re: not understand how my book said pka=ph when its not even at 1/2 equivalence pt
Post by: Borek on October 22, 2014, 02:58:01 AM
Quote from: iammrhappy on October 22, 2014, 02:06:37 AM
Are they saying pka = ph because they are assuming the ion is at 1/2 equivalence point? I'm confused. How did they get pka=ph? Is it a typo? We're not even talking about titrations in this example.

They didn't say pH is 9.24. They said "at pH 9.24 [NH3]=[NH4+]". That's not about pH 7.0 solution, but it is a good starting point to find the concentrations asked about.

Quote
Also I don't understand how they got 1:100 due ph of 7 being 2 units lower then the ph of 9.24. How does that work?

Please see http://www.titrations.info/acid-base-titration-indicators

While it is about indicators, equation (2) holds for any weak acid, and nicely shows how the ratio of the protonated/free form changes with pH (and why it changes tenfold with pH changing by one unit).