How much more acetic acid (pKa = 4.76) than acetate should be in solution to maintain a pH of 4.00?

Question

I need assistance with a chemistry problem related to buffer solutions. Specifically, I am trying to determine how much more acetic acid (with a pKa of 4.76) is needed than acetate to maintain a pH of 4.00. I am confused about the calculations and keep arriving at 0.17, which I believe is incorrect. Could someone please explain how to approach this problem step-by-step? Thank you!

Anonymous · Accepted Answer

Use the Henderson-Hasselbalch equation to find the needed ratio of Ac-/HAc.

pH = pKa + log ([acetate] / [acetic acid])

4.00 = 4.76 + log ([acetate] / [acetic acid])

-0.76 = log ([acetate] / [acetic acid])

10^-0.76 = ([acetate] / [acetic acid]) = 0.174

Inverting that, ([acetic] / [acetate]) = 1 / 0.174 = 5.75

Anonymous · Answer

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How much more acetic acid (pKa = 4.76) than acetate should be in solution to maintain a pH of 4.00?
Help I don't know what to do.. I keep getting 0.17 which is the wrong answer. How can I solve this?

Anonymous · Answer

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Use Henderson-Hasselbalch equation. pH = pKa + log₁₀( [CH₃COO⁻]/[CH₃COOH] ) Since acetic acid is a weak acid, it can be assumed that ionization of the acetic acid and hydrolysis of the acetate ions are negligible. Hence: [CH₃COOH] = 2M and [CH₃COO⁻] = [NaCH₃COO] = (5g / 82g/mol) / 0.01L = 6.1M The pH of this buffer solution is: pH = 4.76 + log₁₀( 6.1 / 2 ) = 5.24

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How much more acetic acid (pKa = 4.76) than acetate should be in solution to maintain a pH of 4.00?

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