TPR workbook Chemistry Passage 34

[tshank]

3. Dilute hydriodic acid (HI) is studied under equilibrium conditions in an aqueous solution. Which of the following ionic forms of HI will predominate in this solution?

A. The conjugate base, I-

B. The conjugate acid, H I+ 2

C. The conjugate base, I-, and its conjugate acid in equal proportions

D. The conjugate acid, H I+, and its conjugate base in

equal proportions


ANSWER (highlight): A


Can someone explain why the answer is not C if the solution is in equilibrium? Thank you!

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[kraskadva]

HI is a strong acid, so it will completely dissociate in aq solution. It would all go to I- (and H+) so A is the only option.
In C, the conjugate base (I-) and its conjugate acid (which is HI, not H+) are given as = concentrations, which doesn't happen.

 

[tshank]

First thought: I thought if the solution was in equilibrium that means I-/HI = 1 Meaning that both I- and HI concentrations are the same.

Second thought:
Or, does I-/HI = K and K is the rate constant also representing keq, capable of being anything, and is representative of equilibrium concentrations?

When does I-/HI = 1? My guess is in more acidic solutions than HI's pka. Correct?

Thanks!

 

[kraskadva]

First thought: I thought if the solution was in equilibrium that means I-/HI = 1 Meaning that both I- and HI concentrations are the same.

No, equilibrium is when the ratio of products:reactants stops changing.

Second thought:
Or, does I-/HI = K and K is the rate constant also representing keq, capable of being anything, and is representative of equilibrium concentrations?

Yes.

When does I-/HI = 1? My guess is in more acidic solutions than HI's pka. Correct?

The pKa of HI is ~ -11pH. If you can get a solution to -11 then you can have a 1:1 ratio of I-:HI.
Otherwise, it'll always be [I-]>>>[HI]

NB: this is for strong acids, of which there are only 6, and you should memorize them.
Weak acids (all others) have much higher pKa's (between the 1-14 scale of normal) and you can easily have situations where pH=pKa so Products:Reactants::1:1

 

[tshank]

Awesome, thank you for the response.