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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: fenor on November 07, 2010, 12:06:14 AM

Title: Finding pH using Activities
Post by: fenor on November 07, 2010, 12:06:14 AM
Accounting for activities, determine the pH of a 0.01 F CH3COOH solution
containing 0.01 F Ca(NO3)2.

Attempt :
(0.01 CH3COOH)(ActivityCoefiictent)
pOH = -log[(0.01 CH3COOH)(ActivityCoefiictent) ]
pH = 14-pOH
Title: Re: Finding pH using Activities
Post by: MrTeo on November 07, 2010, 05:16:55 AM
Quote from: fenor on November 07, 2010, 12:06:14 AM
pOH = -log[(0.01 CH3COOH)(ActivityCoefiictent) ]

Why pOH? CH3COOH is a weak acid... Anyway you also need the dissociation constant for acetic acid (Ka=1.8e-5 if i remember well...)
Title: Re: Finding pH using Activities
Post by: fenor on November 07, 2010, 11:11:44 AM
Quote from: MrTeo on November 07, 2010, 05:16:55 AM
Quote from: fenor on November 07, 2010, 12:06:14 AM
pOH = -log[(0.01 CH3COOH)(ActivityCoefiictent) ]

Why pOH? CH3COOH is a weak acid... Anyway you also need the dissociation constant for acetic acid (Ka=1.8e-5 if i remember well...)
Yes that was my mistake , but where would I use Ka?
Title: Re: Finding pH using Activities
Post by: Borek on November 07, 2010, 12:21:17 PM
http://www.chembuddy.com/?left=pH-calculation&right=pH-weak-acid-base
Title: Re: Finding pH using Activities
Post by: fenor on November 07, 2010, 01:51:30 PM
Ka = [H+] [CH3COO-] / [CH3COOH] = 1.8 x 10^-5
What happens to Ca(NO3)2?
Title: Re: Finding pH using Activities
Post by: Borek on November 07, 2010, 01:53:48 PM
Quote from: fenor on November 07, 2010, 01:51:30 PM
What happens to Ca(NO3)2?

Nothing, it is there just to change ionic strength.
Title: Re: Finding pH using Activities
Post by: fenor on November 07, 2010, 02:04:50 PM
Im really confused here,

Ka = [H+]^2/Ca-[H+]

since CH3COOH = Ca -[H+]

Do I have to find Ca before this?
Title: Re: Finding pH using Activities
Post by: Borek on November 07, 2010, 02:50:10 PM
Ca is ambiguous. But both things that you can mark as Ca are given. And they both equal 0.01M.

Perhaps you should answer the question - what IS Ca? - first.
Title: Re: Finding pH using Activities
Post by: fenor on November 07, 2010, 03:27:28 PM
Ca is equal to HA

Since we need H+

Then H+ = to sqrt ( Ka*Ca)

right?
Title: Re: Finding pH using Activities
Post by: Borek on November 07, 2010, 04:49:58 PM
Quote from: fenor on November 07, 2010, 03:27:28 PM
Ca is equal to HA

No, it equals sum of HA and A-, that is, analytical concentration of acid.

Quote
Since we need H+

Then H+ = to sqrt ( Ka*Ca)

right?

Either right or wrong, depending on the circumstances. Formula you use works only if some condition is meet. You should check it.
Title: Re: Finding pH using Activities
Post by: fenor on November 08, 2010, 01:29:50 PM
That would be right if the 5% rule applies if im not mistaken.
Title: Re: Finding pH using Activities
Post by: Borek on November 08, 2010, 01:47:47 PM
So check if it applies and you will know.
Title: Re: Finding pH using Activities
Post by: fenor on November 08, 2010, 01:57:48 PM
since KA=1.8 *10^-5 is very small then 5% rule applies. So the equation should be right.
Title: Re: Finding pH using Activities
Post by: Fenix on November 08, 2010, 09:07:13 PM
*cough* #2
http://www.chem.ucalgary.ca/courses/f10/chem311/311anmt2f10.pdf (http://www.chem.ucalgary.ca/courses/f10/chem311/311anmt2f10.pdf)
ps. That assignment is mental.