DAT bootcamp chemistry question

[bustakorce1]

I don't understand why C is the correct answer. I look at the solution given but I still don't understand it. Here is the question and answer options.

A small amount of aqueous NH3 is added to a solution of Cu2+ and a precipitate forms. However, when excess aqueous NH3 is added, the former precipitate dissolves. Which statement best explains why the precipitate dissolves with excess aqueous NH3?

• A Cu2+ is reduced to Cu+
• B. Cu2+ is oxidized to Cu3+
• C. Cu2+ forms a complex ion with NH3
• D. NH3 acts as a Bronsted-Lowry base to dissolve the acidic precipitate
• E. The activation energy to react with the precipitate is overcome with excess NH3

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[tooth knockn]

Choice C is correct, why?

I don't know, but I do know this is the only complicated reaction of this kind with excess nh3 with Cu

 

[bustakorce1]

This is the solution given as to why C is correct, I don't understand it.

Complex ions are those such as Cu(NH3)42+. The water molecules that normally surround a cation are replaced with some other electron pair donor (NH3 in this case). With excess ammonia, it is possible to form a complex ion with the copper ion. Complex ions tend to be very colorful as well.

 

[vk223]

Bump! Can someone explain this problem please??

 

[ixyz]

it turns out that when the solution is dilute the Cu2+ reacts with water instead of NH3, forming Cu(OH)2 solid, and when you add xs NH3 it forms a complex ion that is aqueous. check out the attachment

 

[bustakorce1]

Ok so Cu(OH)2 is the precipitate that initially forms. I see. I didn't know hydroxide salts are insoluble

 

[ixyz]

Ok so Cu(OH)2 is the precipitate that initially forms. I see. I didn't know hydroxide salts are insoluble

-OH is only soluble with group 1A alkali metals and Ca2+ Ba2+ and Sr2+ according to the solubility rules, so Cu(OH)2 is a solid in a solution

 

[emminent]

-OH is only soluble with group 1A alkali metals and Ca2+ Ba2+ and Sr2+ according to the solubility rules, so Cu(OH)2 is a solid in a solution

I've been enlightened...