Im stumped on 2 G-chem questions, need help please!

[Hunned Block]

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I've been trying to figure out these two questions all day anyone here care to explain them to me?

Q1

Acetylcholine is a potent neurotransmitter. COncentrations between nerves is regulated by the enzyme acetylcholinesterase. This enzyme degrades acetylcholine to choline and acetic acid. The acetic acid then dissociates to acetate ion and hydrogen ion. Calculate the amount of acetylcholine present in 15 mL if the addition of acetylcholinesterase decreases the pH from 7.4 to 6.8. (the Ka for acetic acid is 1.74 x 10^-5)

The answer is 1.58 x 10^-7...but i have no idea how they got this answer, the question itself is confusing. Im pretty sure the Henderson hasselbach equation was used but how do they get this answer?

Q2

A biochemistry text has the Ka for dihydrogen phosphate as 1.38 x 10^-7 and a Quantitative analysis book has the Ka for dihydrogen phosphate as 6.2 x 10^-8. Explain this apparent discrepancy.

does it have to do with temperature?

Any help would be appreciated.

 

[D.D.S.]

Wow.. I dont think the DAT's Gchem is that detailed, unless it changed.

 

[saltlake007]

Where did you get question 1 from...good lord...I thought I knew everything about chem...guess am wrong.

 

[TheWiredNerv]

If I remember correctly, Ka is dependent upon temperature.
dG = -RT ln Ka
that equation is Gibb's Free Energy. There was probably a temperature difference that rose the Ka.

 

[Montserrat]

I've been trying to figure out these two questions all day anyone here care to explain them to me?

Q1

Acetylcholine is a potent neurotransmitter. COncentrations between nerves is regulated by the enzyme acetylcholinesterase. This enzyme degrades acetylcholine to choline and acetic acid. The acetic acid then dissociates to acetate ion and hydrogen ion. Calculate the amount of acetylcholine present in 15 mL if the addition of acetylcholinesterase decreases the pH from 7.4 to 6.8. (the Ka for acetic acid is 1.74 x 10^-5)

The answer is 1.58 x 10^-7...but i have no idea how they got this answer, the question itself is confusing. Im pretty sure the Henderson hasselbach equation was used but how do they get this answer?

Q2

A biochemistry text has the Ka for dihydrogen phosphate as 1.38 x 10^-7 and a Quantitative analysis book has the Ka for dihydrogen phosphate as 6.2 x 10^-8. Explain this apparent discrepancy.

does it have to do with temperature?

Any help would be appreciated.

Question 1 seems vaguely familiar. I've seen it on one of my problem sets in biochem a few years earlier. Do your own homework.

Question 2: Sure you aren't looking at Ka (as the association constant of enzyme complex to ligand) as opposed to Ka (dissociation constant) ?

From the Van't Hoff equation, you'll know that K will either increase or decrease with temperature changes, depending on whether the reaction is endo/exothermic.

Second, a biochemists standard state is different from a chemists standard state, to account for reactions at physiological conditions. If you see a prime designation after a constant, it's derived from a biochemists standard state.