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Offline zmasterflex

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Buffer Problem
« on: March 30, 2014, 12:27:30 PM »
I'm having trouble with this buffer problem;
What is the pH of a buffer prepared by adding 180 mL of .1M NaOH to 200mL of .1M acetic acid? Ka of acetic acid is 1.8e-5

I'm using Henderson-Hasselbach: pH = pKa + Log(base/acid). I plug in given pKa and I solve for the molar amounts of the titrants (NaOH = .1M x .180mL and H+ using the Ka that's given). My calculated answer is incorrect and I'm thinking that the equation isn't being used properly. When do I use this equation and what do I use in this case? thanks

Offline Borek

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Re: Buffer Problem
« Reply #1 on: March 30, 2014, 01:55:47 PM »
What is the acid here? What concentration of the acid do you plug into HH equation?

What is the base here? What concentration of the base do you plug into HH equation?
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Offline zmasterflex

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Re: Buffer Problem
« Reply #2 on: March 30, 2014, 02:18:09 PM »
The added Base is 180mL of .1M NaOH. This is the base which will have .018 moles. Divided by the 380mL or .380 L then we have the value to plug into the HH (approx .04736). The acid concentration we get from the ICE table. HAc <--> H + Ac-
Ka = [H][Ac-]/[HAc]
Ka = x^2/.1-x we ignore the bottom x because the Ka is so small and we get x^2/.1 = 1.8e-5, x^2 = 1.8e-6 and x = 1.34e-3. This is the Molarity in 200mL so we need to compensate for the new volume. 1.34e-3 x .2L = 2.68e-4 moles. Divide this by new volume (.38L) get 7.05e-4 as the Molarity of acid. Plugging this into HH we get pH = p(1.8e-5) + log(.047/7.05e-4) = 6.56 which is too high, the answer given is 5.7. Am I making a math error, a procedural error, or is this just the wrong use of the equation? Why would we use the concentration of acid and the pKa of the acid but only the concentration of the base? What happens if we have strong acid and weak base? Weak and weak? Do we always use this equation?

Offline Borek

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Re: Buffer Problem
« Reply #3 on: March 30, 2014, 03:12:44 PM »
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Offline zmasterflex

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Re: Buffer Problem
« Reply #4 on: March 30, 2014, 05:18:56 PM »
Reading that is making my brain hurt more than it already was before. Can you please tell me at least if I'm setting up the equation correctly and if the math looks somewhat correct?

Offline Borek

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Re: Buffer Problem
« Reply #5 on: March 30, 2014, 05:43:41 PM »
No, you are setting it incorrectly, which is why I pointed you to the pages where it is explained.
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