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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: helplessnerd0402 on August 23, 2021, 07:11:54 PM

Title: [H+] from pH and molarity
Post by: helplessnerd0402 on August 23, 2021, 07:11:54 PM
I'm working on some practice problems from my prof and I'm stuck on this one:

What is the [H+] of a 1.4 M acetate buffered solution at pH 6.2?

Do I just use Henderson-Hasselbalch by plugging in the pKa for acetate and solving for [HA]? This is for biochem, but it's been three years since I've taken gen chem.
Title: Re: [H+] from pH and molarity
Post by: Babcock_Hall on August 23, 2021, 07:27:07 PM
I would reread the question carefully.
Title: Re: [H+] from pH and molarity
Post by: helplessnerd0402 on August 23, 2021, 07:45:00 PM
Quote from: Babcock_Hall on August 23, 2021, 07:27:07 PM
I would reread the question carefully.

I initially just figured I could use the pH to calculate [H+] by using [H+] = 10^(-pH), but then I thought I was oversimplifying it.
Title: Re: [H+] from pH and molarity
Post by: Babcock_Hall on August 23, 2021, 07:58:44 PM
I think that there is extraneous information in this question.

On a related and more complex matter, if the problem had asked for calculation of the concentrations of acetic acid (HA) and acetate ion (A1-), these could be obtained with the H-H equation, but one would use an apparent pKa at high ionic strength.  At the stated concentration of 1.4 M, the ionic strength is quite a bit higher than for the thermodynamic pKa, which is zero.  Biochemistry laboratory courses sometimes discuss this problem, but it may not be a concern for you.  Others here would know more about this topic than I do, but it is a matter of some practical concern when preparing buffers.
Title: Re: [H+] from pH and molarity
Post by: helplessnerd0402 on August 23, 2021, 09:08:47 PM
Quote from: Babcock_Hall on August 23, 2021, 07:58:44 PM
I think that there is extraneous information in this question.

On a related and more complex matter, if the problem had asked for calculation of the concentrations of acetic acid (HA) and acetate ion (A1-), these could be obtained with the H-H equation, but one would use an apparent pKa at high ionic strength.  At the stated concentration of 1.4 M, the ionic strength is quite a bit higher than for the thermodynamic pKa, which is zero.  Biochemistry laboratory courses sometimes discuss this problem, but it may not be a concern for you.  Others here would know more about this topic than I do, but it is a matter of some practical concern when preparing buffers.
Thanks. This is really just supposed to be review from gen/orgo chem and I would hope it isn't more complex than I initially thought. The irrelevant info just threw me off.