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Topic: Buffer Problem  (Read 2242 times)

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Offline polexia

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Buffer Problem
« on: August 27, 2013, 01:57:06 AM »
Buffer solution is prepared by adding 40mL of 0.250M NaH2PO4 and 25mL of 0.2M Na2HPO4 solution. What is the pH of the resulting solution?

Help appreciated!
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Offline billnotgatez

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Re: Buffer Problem
« Reply #1 on: August 27, 2013, 02:00:11 AM »
Please read forum rules
you should show your attempts
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Offline polexia

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Re: Buffer Problem
« Reply #2 on: August 27, 2013, 08:28:56 PM »
I know that you have to find the concentrations of both compounds in equilibrium and this is what I'm having trouble with. Once I have the concentrations, I use the henderson-hasselbach equation, knowing the pKa to find the pH of the solution.
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Offline Borek

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Re: Buffer Problem
« Reply #3 on: August 28, 2013, 02:51:27 AM »
You can safely assume equilibrium concentrations are just those of the substances introduced into the solution - that is, equilibrium doesn't shift concentrations after preparing the solution.

Compare

http://www.chembuddy.com/?left=buffers&right=composition-calculation

and

http://www.chembuddy.com/?left=buffers&right=with-ICE-table
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ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline polexia

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Re: Buffer Problem
« Reply #4 on: August 29, 2013, 12:58:55 AM »
I used the two values and put them directly into the H-H equation:

pH = 6.37 + log (0.2/0.25)
pH = 6.27

This didn't come up with the right answer?

The correct answer is 6.91
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Offline Borek

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Re: Buffer Problem
« Reply #5 on: August 29, 2013, 03:06:09 AM »
What is pKa2 of phosphoric acid?

(Hint: it is different from pKa1 of carbonic acid).
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