Ionization constant

[752779]

Hey guys, I understand that a weak acid does not dissociates completely. The choices given is very intimidating. Near zero (which can either be greater than zero, but less than 1), about 1 ( can either be less than 1 but above zero). The answer gave much less than one. How can one come to know that H2S value? Or it's always much less than one when it comes to weak acid?

Thanks

---

Members don't see this ad.

 

[aldol16]

In my opinion, this is a poorly constructed question - in the answer choices at least. "Near zero" and "much less than one" can be construed to mean almost the same thing. In other words, for a weak acid, the ionization constant can be either near zero (as in much smaller than one although it can't be less than zero - perhaps it's this minute detail the question turns on) or much less than one.

 

[752779]

In my opinion, this is a poorly constructed question - in the answer choices at least. "Near zero" and "much less than one" can be construed to mean almost the same thing. In other words, for a weak acid, the ionization constant can be either near zero (as in much smaller than one although it can't be less than zero - perhaps it's this minute detail the question turns on) or much less than one.

I totally agree with you on that. Thanks for replying. Big help!.

 

[NextStepTutor_2]

Hey guys, I understand that a weak acid does not dissociates completely. The choices given is very intimidating. Near zero (which can either be greater than zero, but less than 1), about 1 ( can either be less than 1 but above zero). The answer gave much less than one. How can one come to know that H2S value? Or it's always much less than one when it comes to weak acid?

Thanks

View attachment 202683 View attachment 202684

Hi @trandzuyhieu !

This is the reason the AAMC is not called a science test. You must learn to "think" like the AAMC and that takes time, patience, and a whole lot of mistakes along the way. Reason all you want, this was a Q on an AAMC exam for years and if you don't think like them, no amount of complaining will get you points because the AAMC can and will argue they are not wrong, just asking you to think. Here they are asking you for a "first approximation" which is a fancy way of saying an educated guess that is a bit more than a shot in the dark.

You know the compound is a weak acid, so it will dissociate a bit, but not much. Which answer is more accurate (not necessarily all right or all wrong)? Choice B. the AAMC will argue that there is a proper way to interpret each statement. Near 0 can be considered negligible, or practically 0 while much smaller than 1 implies some non-negligible value. Is that how I would speak to my patients or fellow scientists? Nope! But that's the way the AAMC thinks and so you need to learn to think that way too. I always tell my students they have to learn to "bring themselves down to the AAMCs level" when it comes to the type of thinking that they want.

perhaps it's this minute detail the question turns on

After you have spent a few years with the MCAT (may it never happen to you!), you will come to see how often this, not pure science and not necessarily strict logic, is what the AAMC uses to make one answer correct and the others incorrect.

Hope this helps, good luck!