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Topic: buffer with a given pH  (Read 3278 times)

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Offline mol

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buffer with a given pH
« on: February 01, 2011, 04:12:49 PM »
If a buffer with a pH of 10.00 is needed, which of the following can we use?
 ethanoic acid with  Ka= 1.8* 10^-5       or   
 ammonia with Kb = 1.8* 10^-5.

I know that pH and pOH must equal 14. so if 10 is to be added then we r left out with 4 .
The negative log of 1.8* 10^-5  is  4.7(still a closer value to 4). we need either a  weak acid ( ethanoic acid) or a  weak base (as ammonia) . So I am not sure how to choose one out of the above 2...
pls help.

Offline Borek

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Re: buffer with a given pH
« Reply #1 on: February 01, 2011, 06:23:07 PM »
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline AWK

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Re: buffer with a given pH
« Reply #2 on: February 02, 2011, 01:24:16 AM »
For buffers you need two reagents: NH3 and eg. NH4Cl, or CH3COOH and eg. CH3COONa

http://en.wikipedia.org/wiki/Buffer_solution
AWK

Offline mol

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Re: buffer with a given pH
« Reply #3 on: February 02, 2011, 11:25:50 AM »
Yes I know Henderson Hasselbach eqtn. But to use that eqtn I must have the concentrations of the base and acid in hand..Here  I dont know the conc of them. Is it still possible I can use the eqtn to solve this.. can u pls show me how to do that?
Thanks

Offline Borek

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Re: buffer with a given pH
« Reply #4 on: February 02, 2011, 12:01:44 PM »
Buffer works when the concentration of both acid and conjugate base are comparable. Best buffer is the one containing both in identical concentrations. Try to plug this information into HH equation.

eqtn eqtn conc eqtn u pls

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Offline mol

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Re: buffer with a given pH
« Reply #5 on: February 02, 2011, 12:05:05 PM »
I'll try that. Thank you Borek and AWK.

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