DAT GC Tough Question to Ask

[sochun90]

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Hi, here's the problem.

An indicator is red in the undissociated form and yellow in the dissociated form. If a solution were at a very low pH, what color would the indicator appear?

The answer is red because, high concentration of hydrogen ion will shift the equilibrium to the left in the reaction such as: HIN->H+ + IN-.

However, my question is, what about in the case such as KOH->K+ + OH-. If more hydrogen ion is added, it will reduce the hydroxide ion and thus shift the reaction to the right, resulting in the dissociated form.

Could anyone rebut my opinion?

Thanks.

 

[Dathrill2002]

K+ is a spectator ion. So adding more H+ into the solution will neutralize the OH-, not necessarily causing more KOH to dissociate (bc the are not really "associated" in the first place). Its pretty much like having a solution of pure OH when u have a KOH.

 

[Ari Rezaei]

In the case of KOH nothing would happen, but I understand what you're saying. If you had a weak base indicator like:

(InO-)Na+ + H2O <---> InOH + OH-

In this case, if you increase the pH (make it more basic), then you can see you will shift to the left. If you decrease the pH and make it more acidic, then yes, you would consume OH- and pull the reaction the the right, giving whatever color that would be. I also think it's interesting to note what "dissociated and undissociated" would mean for organic bases. I actually wonder what the correct definitions are, would the right side be considered "undissociated"?