How much propanoic acid is deprontonated in water (assuming pKa is 5)?

[UIUCstudent]

The answer is less than 1%.

I originally thought that the answer would be like 99% or something b/c I assumed pH of water to be 7. Therefore since pH is greater than pKa by two, the majority of the species is deprontonated.

But the reason this doesn't happen is because water is a very weak acid/base therefore deprontonation occurs very slowly if at all. You would have to tackle this problem using the ICE method.

Well this is my line of reasoning since the answer BR gave seemed to be too terse for me. Is what I said correct?

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[Renaissance Man]

My reasoning behind this answer is that propanoic acid is a weak acid and therefore does not disassociate 100% (or even close to 100%). The pKa measures an acid's strength (the lower it is the stronger the acid). I don't know the answer choices, but since the Ka of propanoic acid is 10^-5, that means that relatively speaking only 1 molecule of H+ disassociates compared to 100,000 molecules of the original acid.

Because water is a neutral solvent, you can assume that the propanoic acid is only thing affecting pH. At least this is how I see the problem.

 

[loveoforganic]

The answer is less than 1%.

I originally thought that the answer would be like 99% or something b/c I assumed pH of water to be 7. Therefore since pH is greater than pKa by two, the majority of the species is deprontonated.

But the reason this doesn't happen is because water is a very weak acid/base therefore deprontonation occurs very slowly if at all. You would have to tackle this problem using the ICE method.

Well this is my line of reasoning since the answer BR gave seemed to be too terse for me. Is what I said correct?

If water was a buffered solution of pH 7 you'd be right. pH rapidly drops as more propionic acid dissociates

 

[BerkReviewTeach]

If water was a buffered solution of pH 7 you'd be right. pH rapidly drops as more propionic acid dissociates

Spot on as usual!

The answer is less than 1%.

I originally thought that the answer would be like 99% or something b/c I assumed pH of water to be 7. Therefore since pH is greater than pKa by two, the majority of the species is deprontonated.

But the reason this doesn't happen is because water is a very weak acid/base therefore deprontonation occurs very slowly if at all. You would have to tackle this problem using the ICE method.

Well this is my line of reasoning since the answer BR gave seemed to be too terse for me. Is what I said correct?

In this question it is pure water as the solvent, so the propanoic acid dissociates according to its Ka. You need to use icebox, but because it's a simple case, you can just jump to the final line.

HA <=> H+ + A-​

We get the same amount of H+ as A-, and [HA] barely dissociates with such a low Ka, so we can ignore the x. This means we get:

[H+][A-]/[HA] = 10^-5​

• If [HA] = 0.10 (just a hypothetical number for now), then the product of [H+] x [A-] = 10^-6.

This means that [H+] = 10^-3 and [A-] = 10^-3, so the ratio of [A-]/[HA] (the ratio of the dissociated species to the protonated species is 10^-3/10^-1, which is 1/100, or about 1%.

Hope that helps.

 

[UIUCstudent]

Sweet thanks guys