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TBR 2 General Chemistry: Equilibrium Questions

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ajumobim

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1st question: Mercury II cations are MOST soluble in which of the following solutions?
A) 0.10 M NaCl
B) 0.10 M Na2S
C) 0.10 M NaI
D) 0.10 M NaNO3

The book says that we must consider the relative solubilities of HgCl2, HgS, and Hg(NO3)2 and I do not understand how that conclusion is made.

2nd question: Silver chloride is most soluble in which of the following solutions at 25 degrees C?
A) 0.10 M HgNO3
B) Pure water
C) AgNO3
D) NaCl

The book explanation says that silver chloride is most soluble in a solution where Hg+ is present because of complex equilibrium. Can you please write the 2 reactions of complex equilibrium?
 

AwayFromReality

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1st question: In each of those answer choice we have certain ions already present in water. For example in choice A, we have Na+ ions and Cl- ions. Since mercury (Hg2+) is a cation, when added to solution it will try to bind to the anion, in this case Cl- to create the salt HgCl2. If the solubility of HgCl2 is low then the salt will precipitate out of the water. This is why we need to look at the relative solubilities of each answer choice. Now the MCAT might give you the relative solubilities outright, might give you the Ksp values and you would have to find the solubilities using that or you will use the solubility rules. In this case we can only use the solubility rules since no numbers are given. NO3(-) salts are usually very soluble in water which makes the solubility of Hg(NO3)2 salt very high which is why answer choice D is correct.

2nd question: I looked up that question within the chapter (which btw is TBR 3 in my book) and a little bit before they gave you that question they mentioned within the text (under the selective precipitation and ion exchange heading) that HgCl is less soluble than AgCl. Which means if a solution contains both Cl- and Hg+ (Hg in this case is Mercury I) then they both will bind to each other and release from the solution as the solid salt HgCl

So in answer choice A we have a solution with ion concentrations like this :
Equation 1: HgNO3(s)-> Hg(+) + NO3(-)

This means that there are already Hg+ ion in the solution so when we add AgCl in the solution it starts dissociating into ions per this equation :
Equation 2: AgCl(s)-> Ag(+) + Cl(-)

The Cl- from equation 2 will bond with the Hg+ in equation 1 precipitating out as solid.
Equation 3: Hg(+) + Cl(-)-> HgCl(s)

This means that according to Le Chatelier's principle since one of the products of equation 2 (Cl-) is being removed by Hg+ in equation 1 more of the AgCl will dissociate therefore increasing its solubility.

You just have to recognize that HgCl has a lower solubility than AgCl. Looking at the answer choices neither C nor D will work because of the common ion effect and Water molecules won't bind with either Ag+ or Cl- therefore no complex equilibrium can happen. Only choice left is Answer A
 
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