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Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: mol on February 01, 2011, 04:12:49 PM

Title: buffer with a given pH
Post by: mol 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.

Title: Re: buffer with a given pH
Post by: Borek on February 01, 2011, 06:23:07 PM

Do you know Henderson-Hasselbalch equation (http://www.chembuddy.com/?left=pH-calculation&right=pH-buffers-henderson-hasselbalch)?

Title: Re: buffer with a given pH
Post by: AWK 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

Title: Re: buffer with a given pH
Post by: mol 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

Title: Re: buffer with a given pH
Post by: Borek 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.

Quote from: mol on February 02, 2011, 11:25:50 AM

eqtn eqtn conc eqtn u pls

Please read forum rules (http://www.chemicalforums.com/index.php?topic=33740.0).

Title: Re: buffer with a given pH
Post by: mol on February 02, 2011, 12:05:05 PM

I'll try that. Thank you Borek and AWK.