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Identify the metal which undergoes these reactions

  1. Jul 1, 2016 #1

    TT0

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    1. The problem statement, all variables and given/known data
    Screen Shot 2016-07-02 at 11.23.24 AM.png

    2. Relevant equations


    3. The attempt at a solution
    The only thing that helped me was that with excess NaOH, the white precipitate dissolves. The only compounds I know that do this are zinc hydroxide and aluminium hydroxide. However, I don't know how the other reactions will help me decide between the two. Could someone explain this?

    Cheers!
     
  2. jcsd
  3. Jul 2, 2016 #2

    Borek

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    Staff: Mentor

    Check properties of the metals involved. For example - last step is clearly precipitation of the sulfide. What color is a lead sulfide?
     
  4. Jul 4, 2016 #3

    TT0

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    Lead sulfide is black according to google images. So I guess it is eliminated. Magnesium definitely wouldn't be it because it won't form a precipitate as a hydroxide.

    This leaves us with zinc, aluminium and tin. Googling each of the sulfides, I can eliminate tin sulfide as it is not white. I don't really know where to go from here, I tried to look at the properties of the metals but I still don't know. In the last reaction, why is H2S on the top and NaOH on the bottom. Is the H2S a catalyst? I think knowing this might help.

    Cheers!
     
  5. Jul 4, 2016 #4

    Borek

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    Staff: Mentor

    Check that.
     
  6. Jul 4, 2016 #5

    TT0

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    I see, my solubility rules was wrong. Group II hydroxides are only slightly soluble. I got confused because they are also strong bases, yet they are only slightly soluble?

    Magnesium sulfide is soluble and forms hydrogen sulfide and magnesium hydroxide. But as there are already hydrogen sulfide and magnesium hydroxide, I guess ti will precipitate. So magnesium is the answer?

    Thanks!
     
  7. Jul 4, 2016 #6

    Borek

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    Staff: Mentor

    No, as magnesium sulfide is soluble.
     
  8. Jul 5, 2016 #7

    TT0

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    In the first 3 reactions, the amphoteric reactions are happening right? This leaves zinc and aluminium as they are the ones that display these properties. When I googled zinc and aluminium sulfide, it seems both are white compounds. How do you differentiate between them?

    Thanks!
     
  9. Jul 5, 2016 #8

    Borek

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    I don't think you can precipitate aluminum sulfide from water solutions - it decomposes on contact with water.
     
  10. Jul 6, 2016 #9

    TT0

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    I see, so the answer is zinc. Thanks for all the help!
     
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