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Buffer Problem!
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Topic: Buffer Problem! (Read 3314 times)
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K'
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Buffer Problem!
«
on:
March 14, 2013, 11:15:24 AM »
The pH of a 0.10 M HA solution is 3.8.
How many mL of 0.10 M NaOH should be added to 20 mL 0.10 M HA to prepare a buffer with a pH of 4.3?
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Borek
Mr. pH
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Re: Buffer Problem!
«
Reply #1 on:
March 14, 2013, 11:19:17 AM »
You have to show your attempts at solving the question to receive help. This is a forum policy.
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ChemBuddy
chemical calculators - stoichiometry, pH, concentration, buffer preparation,
titrations.info
K'
Regular Member
Posts: 20
Mole Snacks: +0/-3
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Re: Buffer Problem!
«
Reply #2 on:
March 14, 2013, 11:35:59 AM »
Here is my work:
[H+] = 10^(-4.3) = 5.01x10^-5 mol/L
.020 L (5.01x10^-5 mol HA/1 L)(1 L/ 0.10 mol NaOH) = 1.00 L
But I don't know it this is correct.
Thanks.
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Borek
Mr. pH
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Re: Buffer Problem!
«
Reply #3 on:
March 14, 2013, 12:55:07 PM »
No idea what you are doing - it is definitely not correct.
Do you know any equations that are used for buffer pH calculations?
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ChemBuddy
chemical calculators - stoichiometry, pH, concentration, buffer preparation,
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K'
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Posts: 20
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Re: Buffer Problem!
«
Reply #4 on:
March 14, 2013, 01:37:33 PM »
I took the anti log to get [H+] then preceded to get volume required.
I don't think that I need an equation to solve this problem, you guys are the experts here.
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Borek
Mr. pH
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Re: Buffer Problem!
«
Reply #5 on:
March 14, 2013, 01:49:36 PM »
Sorry, but it looks like you have no idea how to do buffer problems. First you need acid Ka, then it is all bout calculating ratio of concentrations of acid and its conjugate base.
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ChemBuddy
chemical calculators - stoichiometry, pH, concentration, buffer preparation,
titrations.info
K'
Regular Member
Posts: 20
Mole Snacks: +0/-3
;)
Re: Buffer Problem!
«
Reply #6 on:
March 14, 2013, 02:09:46 PM »
Actually I think I know about ka and pka etc.
my calculations showed that:
pka = 6.6
ka = 2.51x10^-7
ratio = 2.40x10^-5
But I still think that I don't need an equation to solve that problem.
Can you please give more details.
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Borek
Mr. pH
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Re: Buffer Problem!
«
Reply #7 on:
March 14, 2013, 02:57:12 PM »
Ratio is incorrect. You have neutralized part of the acid - you can safely assume neutralization was stoichiometric and went to completion.
And while there are many ways of skinning that cat, the simplest approach is to calculate ratio and plug it into the
Henderson-Hasselbalch equation
.
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ChemBuddy
chemical calculators - stoichiometry, pH, concentration, buffer preparation,
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K'
Regular Member
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Mole Snacks: +0/-3
;)
Re: Buffer Problem!
«
Reply #8 on:
March 14, 2013, 03:08:27 PM »
I used that equation and I think did a mistake.
Thanks for pointing that out.
after few corrections, I arrived at 5.32 mL required.
I just need your confirmation that is the correct answer.
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Borek
Mr. pH
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Re: Buffer Problem!
«
Reply #9 on:
March 14, 2013, 04:31:45 PM »
Your pKa value is OK, but the volume is wrong.
However, there is something wrong with the question IMHO. pKa 6.6 acid should not be used to prepare pH 4.3 buffer.
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ChemBuddy
chemical calculators - stoichiometry, pH, concentration, buffer preparation,
titrations.info
K'
Regular Member
Posts: 20
Mole Snacks: +0/-3
;)
Re: Buffer Problem!
«
Reply #10 on:
March 15, 2013, 10:25:21 AM »
Thanks for the help
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