18. Another ratio. Calculate the concentration of acetic acid and acetate ion in

Question

18. Another ratio. Calculate the concentration of acetic acid and acetate ion in a 0.2 M acetate buffer at pH 5. The pKa of acetic acid is 4.76. 6

okay so i keep using the hasselbach equation to solve this and im not getting the correct answers. I already know the answers for the acetic acid and the acetate ion. I dont understand what was done to get this answer. Could someone show me step by step to conquer this problem?! it seems the material in biochem builds off each chapter so i do not want to more on until i can figure this out. thx in advance

Explanation / Answer

So starting with the H-H equation, you can calculate the ratio of [acetate]/[acetic acid] 5 = 4.76 + log [acetate]/[acetic acid] [acetate]/[acetic acid] = 1.74 OK, so you know that [acetate] + [acetic acid] = 0.2 M (Whenever the concentration of a buffer is given like that, that means that the total concentration of the weak acid and its conjugate base is that concentration.) So, you have two equations and two unknowns: [acetate]/[acetic acid] =1.74 [acetate] + [acetic acid] = 0.2 M You should be able to do basic algebra to get to an answer. What I do is rearrange the first one to give: [acetate] = 1.74[acetic acid]. Then substitute that into the second one to give: 1.74[acetic acid] + [acetic acid] = 0.2 2.74[acetic acid] = 0.2 [acetic acid] = 0.07 M [acetate] = 0.2 - 0.07 = 0.13 M

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