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Topic: Buffer questions please help  (Read 6051 times)

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Offline jokulsa

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Buffer questions please help
« on: October 05, 2014, 11:00:02 AM »
1. How many moles of HF (pKa = 3.14) must be added to 500mL of 0.30M sodium fluoride (NaF) to give a buffer of pH 3.5? Ignore volume changes due to addition of hydrofluoric acid.



2. What is the molarity of CH3COO- in a buffer with a final pH of 5.20 if the concentration of CH3COOH is 0.25M? (pKa of CH3COOH is 4.7)


thanks

Offline Hunter2

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Re: Buffer questions please help
« Reply #1 on: October 05, 2014, 11:15:33 AM »
Do you know Henderson Hasselbalch equation

Offline jokulsa

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Re: Buffer questions please help
« Reply #2 on: October 05, 2014, 11:19:56 AM »
yeah I know it.
I think I managed to work out the second one, I don't really know where to start on the first..

Offline Hunter2

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Re: Buffer questions please help
« Reply #3 on: October 05, 2014, 11:22:17 AM »
The first one is similar
develop HH-equation for it

Offline jokulsa

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Re: Buffer questions please help
« Reply #4 on: October 05, 2014, 11:25:46 AM »
Is the 500mL given in the first question just there to confuse you?

If so, would the solution be

3.50 = 3.14 + log(0.30/[acid]) 

and then finding [acid] mathematically?

Offline Hunter2

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Re: Buffer questions please help
« Reply #5 on: October 05, 2014, 11:27:48 AM »
Yes, but the value is wrong what does 0,3 M means

Offline jokulsa

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Re: Buffer questions please help
« Reply #6 on: October 05, 2014, 11:34:47 AM »
Isn't this the [base]?

Offline Hunter2

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Re: Buffer questions please help
« Reply #7 on: October 05, 2014, 12:01:07 PM »
M = molarity it has the unit mol/l, so how much mole you have in 500 ml

Offline jokulsa

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Re: Buffer questions please help
« Reply #8 on: October 05, 2014, 12:11:26 PM »
I believe you would have 0.15 moles in 500mL

Why do you need to know this for this question?

Offline Arkcon

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Re: Buffer questions please help
« Reply #9 on: October 05, 2014, 12:14:06 PM »
Write out the Henderson-Haselbach equation for us.  Then write it again, with the correct values in the correct places.  We'll look and see where you might be getting it wrong.  That's how this forum works: http://www.chemicalforums.com/index.php?topic=65859.0
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline Hunter2

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Re: Buffer questions please help
« Reply #10 on: October 05, 2014, 12:23:35 PM »
I believe you would have 0.15 moles in 500mL

Why do you need to know this for this question?

You have HH equation you have pH, pka and the amount of the moles of salt, now develop the equation please.

Offline jokulsa

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Re: Buffer questions please help
« Reply #11 on: October 05, 2014, 10:36:21 PM »
the HH equation is

pH = pKa + log([base]/[acid])


using the values I have,


3.50 = 3.14 + log([base]/[acid])


I'm sorry but I still don't understand where and how the moles of NaF come into it.

Offline Hunter2

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Re: Buffer questions please help
« Reply #12 on: October 06, 2014, 12:41:45 AM »
What is the base and what is the acid here.

Offline jokulsa

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Re: Buffer questions please help
« Reply #13 on: October 06, 2014, 02:25:10 AM »
HF is the acid and NaF is the base

Offline Hunter2

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Re: Buffer questions please help
« Reply #14 on: October 06, 2014, 03:40:25 AM »
Yes correct. So now add the values into the HH equation.

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