UWorld Gen Chem Question

[avalonisland888]

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One of the questions asks how to titrate sebacic acid and the answer is to dissolve it in a base and then use acid to titrate. I'm just wondering...why do we dissolve in a base if it's an acid? For all titration questions, do we always dissolve in the opposite solution e.g. if we wanted to titrate a base, we first dissolve in acid?

 

[rainbow_aloe_vera]

I am assuming that this the question, I saw the same one on reddit.

The reason why you add a strong base is to ensure that the sebacic acid completely dissolves in water (deprotonating the hydrogen on COOH groups will produce COO- which is freely soluble in water, as ions are). Sebacic acid is only slightly soluble in water and so to ensure that it dissociates completely a strong base is added. You can tell that it is only slightly soluble from the aliphatic portion of the molecule consisting of a longggg hydrocarbon chain. The strong base KOH definitively deprotonate the most acidic Hs which are the ones on the carboxylic acid functional groups. The other solvents won't do this as they are all acids/salts ( CH3COOH is a weak acid, HCl is a strong acid, KCl is a salt etc). After dissolution, the sebacic acid will be in equilibrium as it is a weak acid, constantly dissociating and reforming etc. After it is dissolved you can add an acid and indicator and perform the titration. I kind of think of the dissolution as the main rate determining step, once you get over the hurdle you can perform the titration to completion.
Hope that helps!

 

[JimKimSlim]

This is unrelated to the question, but that is a great problem. Definitely expect to see a problem like this on the actual test.

 

[avalonisland888]

Thank you for your explanation!! It makes sense now