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Bootcamp - Gen Chem Test 4 - #16

moose786

Full Member
5+ Year Member
Jan 25, 2016
46
19
  1. Pre-Dental
    Hey guys,

    So I was going over the video explanation for #16 on Bootcamp for Test 4 Gen Chem. I hope I'm not violating any rules, but I'll post just the question here:

    "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?"

    In the explanation, he first says that there will be a precipitate formed in dilute NH3, but then at the end of the video when he lists his bullet-summary, he says the exact opposite "No precipitation occurs with our dilute NH3 conditions". He does the same with the excess NH3 explanation, too.

    Can anyone help clarify where my thinking is flawed? Or does he really contradict himself? It's 99.9% me, but I just need a differently worded explanation, I suppose, since I can't seem to grasp the concept.
     

    ath94

    Full Member
    Jul 1, 2016
    28
    5
    1. Pre-Dental
      It's quite confusing in his explanation. But in general, ammonia in a large amount can form a complex with Cu(OH)2 thus dissole it. Meanwhile, in a dilute ammonia, there is not enough NH3 to do this. (The complex is usually Cu(NH3)4(OH)2 or in its hydrate form)

      You can read this website if you want more
      http://www.chemguide.co.uk/inorganic/transition/copper.html
       
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