Use the Henderson-Hasslebalch equation to calculate the volume of 0.20M NaOH required to adjust the pH of the 0.10M acetic acid solution to the pKa value of acetic acid. Note: you will first need to calculate the number of moles of NaOH and then use the concentration of the NaOH to determine the corresponding value.

 

I do not understand where to start since to find number of moles of NaOH you need the volume but the volume was not given.

 

I do not understand where to start since to find number of moles of NaOH you need the volume but the volume was not given.

Have you read the question carefully, it ask you to find the volume? Let me give you a little push use V as the volume and do all the calculation.

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Use the Henderson-Hasslebalch equation to calculate the volume of 0.20M NaOH required to adjust the pH of the 0.10M acetic acid solution to the pKa value of acetic acid. Note: you will first need to calculate the number of moles of NaOH and then use the concentration of the NaOH to determine the corresponding value.

 

I do not understand where to start since to find number of moles of NaOH you need the volume but the volume was not given.

 

Start with the things that you know. The Henderson-Hasselbach equation:

 

pH = pKa + log to base 10 ([A-]/[HA])

 

where [HA] is the molar concentration of the undissociated weak acid and [A-] is the molar concentration of the acid's conjugate base

 

pKa = -log to base 10 (Ka)

 

where Ka is the acid dissociation constant

 

i.e. pKa = -log to base 10 (([H30+] + [A-])/[HA])

 

The reaction:

 

Sodium hydroxide + acetic acid -> sodium acetate + water

NaOH + CH3COOH -> CH3COONa + H20

 

pH = -log [H+]

 

pKa = pH (the equivalence point) when exactly 1/2 of the acid has dissociated

 

pKa (acetic acid) = 4.76

 

Here is a useful resource by Wiley Essential Biochemistry, for understanding pH, features self-test exercises and animations: http://www.wiley.com/college/pratt/0471393878/student/review/acid_base/index.html

Edited September 24, 201312 yr by Tridimity