acid base question in EK 1001

[Spiker]

q797
Hydroiodic acid has a much higher ka than hydrochloric acid, in aqueous solution equal concentration of each produce essentially the same pH. However, in acetic acid solution, hydroiodic acid produces a power pH. Which of following is the most reasonable explanation for this obervation?

A. Hydroiodic acid is a stronger acid than acetic acid.
B. the conjugate base of hydrochloric acid is a stronger base than water.
C. Acetic acid is a weaker base than water
D. Acetic acid is a stronger base than water.

C is the correct answer. In aqueous solution, water eagerly accepts a proton from either HCl or HI and we get total dissociation. Since HI produces more H+ in the acetic acid solution. HCl must not totally dissociate in acetic acid. This can only mean that acetic acid is less willing to accept protons than water. Accepting proton is a basic quality. Water is a stronger base than acetic acid. Choice A is true, but doesn't explain anything. Choice B is false...

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How exactly does choice C affect HCl more than HI? What I was thinking is that I- is a better leaving group than Cl-

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[Spiker]

And also the conjugate base of strong acid are consider to be neutral?

 

[Dr Gerrard]

Since acetate (the conjugate base of acetic acid) is a weaker base than water, it is less likely to accept protons.

HI is a stronger acid that HCl (although both are classified as strong acids, HI, if I recall correctly, is one of the strongest acids, if not the strongest).

Since acetate is less likely to accept protons, the stronger the acid, the more it will be forced to accept a proton.

If you lower the pH of the solution, there are more hydrogen ions present in acetic acid solution than in water, thus the forward reaction of HCl and HI are less favored.

I think basically, although the change might not be very large, it does exist a little bit, which is what the problem was testing.

Hope that helps, I tried to say it in a couple of different ways because I had a hard time thinking of the best way to explain it.

 

[Will Hunting]

q797
Hydroiodic acid has a much higher ka than hydrochloric acid, in aqueous solution equal concentration of each produce essentially the same pH. However, in acetic acid solution, hydroiodic acid produces a power pH. Which of following is the most reasonable explanation for this obervation?

A. Hydroiodic acid is a stronger acid than acetic acid.
B. the conjugate base of hydrochloric acid is a stronger base than water.
C. Acetic acid is a weaker base than water
D. Acetic acid is a stronger base than water.

C is the correct answer. In aqueous solution, water eagerly accepts a proton from either HCl or HI and we get total dissociation. Since HI produces more H+ in the acetic acid solution. HCl must not totally dissociate in acetic acid. This can only mean that acetic acid is less willing to accept protons than water. Accepting proton is a basic quality. Water is a stronger base than acetic acid. Choice A is true, but doesn't explain anything. Choice B is false...

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How exactly does choice C affect HCl more than HI? What I was thinking is that I- is a better leaving group than Cl-

Ok, from here on out realize that the point of the MCAT is to get the answer. When I began I would get it to A or C and feel C but pick A because I couldn't scientifically justify the choice.

Acetic Acid is a carboxylic acid with is a stronger acid than H20. So, it's conjugate base is weaker than H20. Then, what EK does well and the AAMC does too, many times the right answer will have a common denominator so it's either C or D.

Now, since HI has a higher ka this means it is a much stronger acid. So, it'll dissociate the best. Now, since acetic acid is a weaker base, it's not going to absract protons as well as H20. So, now, the difference in acidity is CRUCIAL. In h20, water is a strong enough base to abstract the weaker H+ from HCl so this is what allows a similar pH for both. HOWEVER, in acetic acid since not as much H+ from HCl- can be pulled off so there is a difference in acidity. HTH

 

[Spiker]

Ha got it. I knew HI>HCl i just read the "higher Ka" wrong...Higher Ka=more acidic right ? with all this log stuff I got momentarily confused.

 

[Will Hunting]

Ha got it. I knew HI>HCl i just read the "higher Ka" wrong...Higher Ka=more acidic right ? with all this log stuff I got momentarily confused.

Yes, be very careful. I made the same mistake!! However, I realized that I is a better acid since it's a larger atom and can better stabilize the negative charge. Remember, bigger ka= better acid and lower pka= better acid. Well done.