home > q&a board > chemistry > topic: molar ratio for buffer solution-please help!

Q BgQuestion:

Scholar
Karma Points: 223
Respect (97%):
posted by  jamieneutron on 4/27/2008 10:29:08 PM  |  status: Closed  

molar ratio for buffer solution-please help!

Course Textbook Chapter Problem
General Chemistry N/A N/A N/A
Question Details:
you are asked to go to the lab and prepare an acetic acid-sodium acetate buffer solution with a pH of 4.00. What molar ratio of CH3COOH to CH3COONa should be used?

I can't figure this one out. Thanks!
Bonus Point Alert! Earn +4 additional karma points for helping this annual member.

AAnswers:

Answer Question
Oracle
Karma Points: 13,472
posted by CHEMNATION! on 4/28/2008 12:21:21 AM  |  status: Live
Asker's Rating: Lifesaver   
jamieneutron's comment:
"thank you!"
Response Details:
pH = pKa + log [base]/[acid]
Ka for acetic acid = 1.8E-5, pKa = 4.74
 
4.00 = 4.74 + log [CH3COO] / [CH3COOH]
The ratio in the formula here is the inverse of what you want, therefore
 
4.00 = 4.74 - log [CH3COOH] / [CH3COO]
-0.74 =  - log [CH3COOH] / [CH3COO]
0.74 =  log [CH3COOH] / [CH3COO]
 [CH3COOH] / [CH3COO] = 100.74 = 5.5
 
This just says you have 5.5 times as many acid molecules present than acetate ions......which should correlate with a pH < pKa......which indeed is true.
Spending precious time on a problem that results in a rating of "not helpful" or "somewhat helpful" is deemed a wasted effort. With certainty, I'll be sure not to provide assistance in that direction again.
 
PROBLEMS WITH CHEMICAL FORMULAS WILL BE ANSWERED FASTER IF PROPER CRAMSTER TOOLBAR FORMATS ARE USED.
 
Answer Question
Ask New Question

Join Cramster's Community

Cramster.com brings together students, educators and subject enthusiasts in an online study community. With around-the-clock expert help and a community of over 100,000 knowledgeable members, you can find the help you need, whenever you need it. Join for free today »

Need Help In

Useful Links