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Homework Help: I need help with general unknown (qualitative analysis)

  1. Nov 25, 2008 #1
    1. The problem statement, all variables and given/known data
    this is the second general unknown in lab.
    my prof. told me i can have 1 or cations and 1 or 2 anions.


    2. Relevant equations



    3. The attempt at a solution
    cation analysis: i carried out blanket tests by adding na2co3, naoh, and nh4oh to my salt dissolved in water to three different test tubes to see if a precipitate formed. a precipitate formed for all three (so i am not able to eliminate any cations from these blanket tests)

    anion analysis: i added agno3 to the salt solution and a white precipitate formed, which says that br-, i-, s2-, po43-, cro42- are absent. i then tried to dissolve the ppt with hno3 and it did not dissolve and according to the test it says i have (cl-, br-, i-, and s2-)

    one last note: i get a green solution when i mix my unknown salt with water, and my text says this: "Suppose a salt mixture dissolves readily and completely in cold water to give a green solutiom. A green solution means the presence of copper, nickel, iron (II), or chromium. the mixture cannot, therefore, contain carbonates, sulfites, phosphates, borates, chromates, or sulfides.

    i tried to test for group 3 cations (fe, co, ni, mn, al, cr, and zn) but i didn't get anything (but i also think i could have made a procedural error)


    any guess at what i may have????? thank you
     
  2. jcsd
  3. Nov 25, 2008 #2

    chemisttree

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    Gold Member

    You are going to have to share the entire procedure with us.
     
  4. Nov 25, 2008 #3
    which one?
    ok ill try again.

    i have an unknown soluble salt. when i add 20 drops water, it turns green.

    i tried the 3 blanket tests with the salt to see if i can eliminate any cations. all blanket tests have failed and a ppt came out ( blanket tests reagents: na2co3, naoh, and nh4oh.)

    when i tried to do an anion analysis (mixing green solution with agno3) i get a white ppt
    i try to dissolve the same ppt with hno3 and it does not dissolve.

    i tested for group 1 (silver, mercury lead) by adding the group reagent (hcl) and got no ppt. this means i have no group 1 cations.

    i tried group 3 test and nothing came out although i think i messed up because i had a ppt at first, but it went away later on.

    i tried a flame test and nothing happened
     
  5. Nov 26, 2008 #4

    Borek

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    Staff: Mentor

    What green cations do you know?

    All "blanket tests" had one thing in common - you have added solutions with high pH.

    And I think anion should be more or less obvious.
     
  6. Nov 26, 2008 #5
    ok....i tested positive today for chlorine! that eliminates group one cations.

    the green cations can be copper, nickel, iron (II), or chromium

    side note: i was separating and detecting bismuth (to get to copper) and in doing so, i got a ppt after adding naoh and sncl2. bismuth says it is a jet black ppt......the ppt was white...should i ignore this even though there was a ppt? thanks!!!!
     
  7. Nov 26, 2008 #6
    when i was testing for acetate.....the confirmatory test was pretty vague, the text says it smells like a fruity nail polish substance.....it smelled like ethanol so i couldn't tell. it also said that you can try an alternative test to see if the salt "charrs" and you smell something like burning hair. it kind of smelled like this so i don't know if i have acetate.
     
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