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Post by mshah » Tue Dec 05, 2017 11:44 pm

Having a hard time with #24 of the C/P of the Official Guide questions. It is not letting me upload a screenshot. I am confused on why the answer isn't B?
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Re: c/p

Post by NS_Tutor_Andrew » Thu Dec 07, 2017 2:18 pm

Hi mshah,

Thanks for the question -- this is definitely one that a lot of students get confused about. In order to understand this, let's work through why D is correct first. I'm going to use "X" to refer to the stearate chain and "XH" to refer to its protonated version, stearic acid, just to avoid typing out C17H32COO.

Per reaction 1, the precipitate is MX, and our goal is to dissolve that. Even an "insoluble" compound with a tiny Ksp will have some of its components in solution, so what we want to do is to find something that will react with M2+ or with X- in a way that will "get rid of it" in order to push the dissolution reaction forward through Le Chatelier's principle. HCl will do the trick perfectly by forming XH. This will push the dissolution reaction forward AND give us a nice weak acid to titrate with strong base.

Now what about NaOH? Theoretically the OH- could interact with M2+ to form M(OH)2 compounds. The thing is that at least for two of the M2+ ions (Ca2+ and Mg2+), M(OH)2 is soluble, so we'd still just have a few M2+ ions floating around, and it wouldn't really push the dissolution reaction forward. For Fe2+, we would get an insoluble hydroxide, so that could conceivably work, but it's not a general solution for all M2+ ions that the passage discusses.

Hope this helps!
Andrew D.
Content Manager, Next Step Test Prep.
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