Homework Help: Chemistry(Please help)

Posted by Hannah on Monday, April 9, 2012 at 4:24pm.

For an acetic acid-sodium acetate buffer to have a pH of 4.64, what molar ratio
of CH3COOH to Na+ CH3COO– is needed?

I know that the Ka value for acetic acid is 1.8e-5 and that I have to divide this value by the value of sodium acetate but I do not understand how you get the value for sodium acetate.

Chemistry(Please help) - DrBob222, Monday, April 9, 2012 at 5:28pm
You want the molar ratio. The easy way to do that is to use the Henderson-Hasselbalch equation.
pH = pKa + log (base)/(acid)
4.64 = 4.74 + log(acetate)/(acetic acid)
Solve for the ratio of
acetate/acetic acid.
Chemistry(Please help) - DrBob222, Monday, April 9, 2012 at 5:31pm
The other way to do it is
Ka = (H^+)(Ac^-)/(HAc)
Ka/(H^+) = (Ac^-)/(HAc)
1.8E-5/2.29E-5 = (Ac^-)/(HAc)
Chemistry(Please help) - Hannah, Monday, April 9, 2012 at 6:19pm
For the second way, where did the 2.29e-5 come from?
Chemistry(Please help) - DrBob222, Monday, April 9, 2012 at 10:42pm
pH of 4.64. (H^+) = 2.29E-5

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