Homework Help: AP Chemistry

Posted by Ana on Monday, January 3, 2011 at 10:20pm.

We're doing a lab in my class and we're supposed to make a buffer at a certain pH.

I have decided to make a buffer with a pH of 5.0 using glacial acetic acid (pure CH3COOH) and sodium acetate.

I found that the ratio of acid to base is 1.41 M: 1.0 M and now I'm stuck.

My teacher only has 17.4 M glacial acetic acid and we basically have to dilute it in 75 mL of water. And I'm confused at this point. Help me please with a step by step description on how to do this?

AP Chemistry - DrBob222, Monday, January 3, 2011 at 10:36pm
Are you using the Henderson-Hasselbalch equation or just Ka for CH3COOH? And show me how you obtained 1.41:1.00M. I can't get that.
AP Chemistry - Ana, Monday, January 3, 2011 at 11:06pm
(pure CH3COOH) Ka = 1.8 x 10-5
sodium acetate NaCH3COO

CH3COOH --> CH3COO + H

Ka= [H+][CH3COO-]/[CH3COOH] = 1.8 x10^-5
[H+]= 4.85

pH = pKa + log [base]/[acid]
5.0 (chosen pH of buffer) = 4.85 + log [base]/[acid]

.15 = log [base]/[acid]

10^0.15 = [1.41 M]/[1 M]
AP Chemistry - DrBob222, Monday, January 3, 2011 at 11:41pm
OK. But the acid:base ratio is not 1.41:1. If pH you want from the problem is 5.0, then (H^+)= 1E-5 and
Ka = (H^+)(CH3COO^-)/(CH3COOH) then
(CH3COOH)/(CH3COO^-) = (H^+)/Ka
acid/base = (1E-5/1.8E-5) = 0.555. But let me show you a much easier way to do it since you typed in the HH equation.
pH = pKa + log (base/acid)
5.0 = 4.745 + log B/A
0.255 = Log B/A
B/A = 1.80 or A/B = 0.555 but I never use either of those as a number although the calculator calculates that number during the following.

Now, how many mL of the 17.4 stuff will you use? 17.4 M x # mL used = ?? millimoles. Then plug that into the HH equation and solve for base (which will be in millimoles if you used acid in millimoles).
5.00 = 4.745 + log(mmoles base)/(mmoles acid) and solve for mmoles base. I get something like 3.13 mmoles base. Convert mmoles to moles, then to grams of CH3COONa. To prepare the buffer you measure the acetic acid with a pipet (or graduated cylinder probably is good enough), add in the grams sodium acetate, stir until dissolved and you have it. Your instructor may have put some limitations on the preparation; for example, you may know how much acetic acid is to be used.

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