O Level Chemistry: Qualitative Analysis (QA)


Another question sent to sean@simplechemconcepts by ‘Student X’ to be shared with all “chemists”. Go work it out!

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- Sean Chua

Question:
When a colourless solution S was added to aqueous sodium hydroxide, no visible change was observed. When aluminum powder was added, a pungent gas which gave a brown precipitate when bubbled into aqueous iron (III) chloride was obtained. What ions could be present in S?

  • A. Na+ , NH4+
  • B. K+ , NO3-
  • C. Na+ , Br-
  • D. Ca2+ , NO3-

Student X’s suggested answers:
What I think: B and D is out as a white ppt is formed with aq NaOH. Hence, either A and C. But I don’t know which one to choose between the two.

PS: Check out “Comments” below for suggested answers.

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12 Responses to O Level Chemistry: Qualitative Analysis (QA)
  1. Gary
    March 23, 2008 | 8:51 pm

    The answer should be A.

    I think the clue given is actually pungent gas.Ammonia will give off a pungent gas.I think the trick is the brown PPT.I don’t think Bromine can never exist as a solid.

    Is my argument valid?

    Reply

    sean Reply:

    Hi Gary,

    Check my suggested answers and reasoning below.

    To Your Academic Success in Chemistry,
    Sean Chua
    Master Trainer & Author
    WINNERS Education Group

    “Experience Learning with A Difference”

    Reply

  2. Gary
    March 23, 2008 | 8:52 pm

    Bromine can ever exist as a solid*

    Sorry for the error

    Reply

  3. grace
    October 4, 2008 | 10:30 am

    Where did the water come from?

    Reply

    sean Reply:

    Hi Grace,

    The water come from “aqueous solutions”.

    Hope this information helps.

    Check my suggested answers and reasoning below.

    To Your Academic Success in Chemistry,
    Sean Chua
    Master Trainer & Author
    WINNERS Education Group

    “Experience Learning with A Difference”

    Reply

  4. Lucas
    March 8, 2009 | 1:55 am

    Hi, I was trying to find out if K+ ions do form a ppt with NaOH. From what I understood, all Group 1 salts are soluble and thus potassium salts should be soluble as well and thus reaction with NaOH will not give a ppt.

    Secondly, the fact that the aluminium was added hints that nitrate ion is reduced to NH3 gas.

    Therefore, I couldn’t decide between A and B since both seemed possible to me.

    Reply

    sean Reply:

    Hi Lucas,

    This is indeed a very tricky question. (I was tricked to believe a wrong answer also).

    Your arguments is correct, no flaws in it.

    Check out my suggested answer and reasoning in the Comments below.

    PS: Drop me a line or two. I would love to hear from you.

    To Your Academic Success in Chemistry,
    Sean Chua
    Master Trainer & Author
    WINNERS Education Group

    “Experience Learning with A Difference”

    Reply

  5. Salman
    March 10, 2009 | 11:57 pm

    Well. Acording to me it does not form a ppt cuz it will form koh which is a base.

    Reply

    sean Reply:

    Hi Salman,

    You are spot on. This is a tricky question, and i was tricked to believe the incorrect answer. =(

    Anyway, if you do not know the correct answer to this MCQ questions, please refer to my suggested answer and reasoning below.

    Cheers,

    Sean Chua
    Master Trainer & Author
    WINNERS Education Group

    “Experience Learning with A Difference”

    Reply

  6. nazri
    March 14, 2009 | 5:21 pm

    yeah answer is A

    Reply

    sean Reply:

    Hi Nazri,

    This is a tricky question, and i was tricked to believe the incorrect answer. =(

    The answer is in fact (B).

    Anyway, if you do not know the correct answer to this MCQ questions, please refer to my suggested answer and reasoning below.

    ps: i would love to hear from you. Drop me a line or two.

    Cheers,

    Sean Chua
    Master Trainer & Author
    WINNERS Education Group

    “Experience Learning with A Difference”

    Reply

  7. Sean Chua
    April 24, 2009 | 12:58 pm

    Hi Everyone,

    This is indeed a very tricky MCQ questions. You need to have a good grasp on Qualitative Analysis (QA) for both Identification of Cations as well as Anions in order to solve it.

    Good approach if you start by using Elimination Strategy to work towards the final answer. Very frequently in QA type questions, we need to use the approach of Elimination.

    My Suggested Strategy:

    Look @ the possible reactions & possible conclusions, Step-by-Step:

    Step 1:

    Colourless Solution S + NaOH(aq) –> No Visible Change observed

    (D) is out since Ca2+ ion forms a white ppt with NaOH(aq).

    We are left with (A), (B) and (C). All S.P.A cations do not form precipitate with NaOH(aq). In fact, they formed soluble products.

    Step 2:

    “When Al is added, a pungent gas is produced”
    – this seems to show that NH3(g) is produced.

    And based on what you have learned before, NH3(g) is produced when NaOH(aq) and Al foil is added to NO3- ion, and upon warming.

    This conclude that NO3- anion is present in the Colourless Solution S.

    Step 3:

    This shows that answer is (B).

    We shall double check on it. KNO3 is indeed a colourless solution, since ALL NITRATES are soluble in water.

    NOTE:

    The brown PPT formed is due to 3 steps:

    1) NH3(g) + H2O(l) –> NH4OH(aq) [recently we call it aqueous NH3]
    2) NH4OH(aq) dissociates to form NH4+(aq) + OH-(aq)
    3) Reaction between the Fe3+ ions present in FeCl3 salt solution with the OH-(aq) from step (2)

    Fe3+(aq) + 3OH-(aq) –> Fe(OH)3(s)

    Fe(OH)3(s) is the Reddish Brown PPT.

    Hope this is clear. Let me know if you need further clarification.
    We can discuss over it.

    To your Academic Success in Chemistry,
    Sean Chua
    Master Trainer & Author
    WINNERS Education Group

    “Experience Learning with A Difference”

    Reply

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