O Level Chemistry & IP Chemistry Notes by 10 Year Series Author - Chemistry Specialist

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O Level Chemistry Questions – Qualitative Analysis (QA) / SPA Tips

Interesting question sent in by another student:

From the scan in the attachment,- How do you answer question 20? How do you know how the different indicators will behave when they are mixed together?-

What’s the correct answer for Q21 and Q22.

Some potassium carbonate solution is added to a sample of tap water. The mixture then appears cloudy. Which of the following ions is probably present in the sample?

A) Bromide ions

B) Magnesium ions

C) Ammonium ions

D) Sulphate ions

Yea, I have no clue how to do this question

What is observed when hydrogen sulphide gas is reacted with iron(III) chloride solution?

What happens when iron is heated with sodium oxide?* Hmm, for this question, isn’t there like no reaction. Cause iron is less reactive than sodium right, so it won’t displace sodium & won’t react? Evidently I’m wrong cause it’s a 5 mark question though. How?

I cant seems to understand Chemistry even i have attended Chemistry Tuition as well as having Chemistry Tutor. It was recommended by a relative, but does not work at all.

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Sean Chua
Many know me as the Ten Year Series book author for JC A-Level H2 Chemistry and O-Level Pure Chemistry. My 19+ years of coaching experience (since 1999) with more than 1500 students from 180+ JCs and Secondary Schools has allowed me to understand the true reasons why students are not able to perform well in Chemistry. Most importantly, my strength lies in using everyday analogies (even grandmothers can understand!) to simplify abstract concepts. Also, my teaching methodology has been designed to be fun, enjoyable and effective for my students. If you want to have an enjoyable time learning Chemistry and being motivated to excel in Chemistry, contact me today at 98287357

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  2. Q21) Answer is Yellow color. At ph5, Phenolphthalein will remain colorless, Methyl orange will be yellow and bromothymol blue will be yellow too. As such, with equal volume, the overal color will be yellow in color for the solution.

    Q22) See below for all 4 choices and answer through elimination:
    A) Sodium carbonate will be formed from carbon dioxide reacting with NaOH, which is soluble in water. As such, no Visible Change will be observed,
    B) Chlorine gas will react with Potassium Iodide (Colorless solution) to form Iodine (Brown) and Potassium Chloride. In this reaction, Iodide ions are oxidized to form Iodine gas. Visible color change from colorless to Brownish solution.
    C) Ammonia gas reacts with Hydrochloric acid to form ammonium salt, in this one -NH4Cl, which is soluble in water due to their ionic nature, As such, no visible change.
    D) No known reaction mechanism between SO2 and Iron (II) Sulphate.
    Thus answer should be (B).

    Q23) For Potassium Carbonate solution reacting with Sample of Tap Water. Question is testing you on the ions that is present in the tap water.
    Potassium carbonate are soluble in water and thus looks colorless, as in all solutions.
    Do elimination again:
    Bromide ions -> formed KBr which is soluble -> no visible change
    Magnesium ions -> formed MgCO3 which is insoluble in water – Visible change with precipitation
    Ammonium ions – > formed NH4CO3 which is soluble in water -> no visible change
    Sulphate ions -> formed K2SO4 which is soluble in water -> no visible change
    Thus answer should be (B).

    Q24) For H2S reacting with FeCl3 solution,
    It involves the reduction of Iron(III)Chloride with Hydrogen sulfide.
    It reduces the yellow acidified Iron(III)Chloride to green Fe2+ with the precipitation of Sulfur.

    Q25) On this question: Iron + Sodium Oxide, do you have more information as in the reaction paramaters, conditions, etc?? Bcos some reactions will occur in controlled high temp, pressure conditions.
    Do let me know more details. Theoretically, at room temp and without any additives – i do not see them reacting.

  3. HAHAHAHAHA. IM a ALEVEL student! not O! LOL. Anyway, nice! Wish there was this blog 2 years ago. =). Any tips for a alevel student?

  4. What is observed when hydrogen sulphide gas is reacted with iron(III) chloride solution?

    H2S(g) + 2FeCl3(aq)-> 2FeCl2(aq) + 2HCl(aq) + S(s)

    so light yellow deposits will be seen. also, the reddish-brown solution changes colour to become a green solution.

    What happens when iron is heated with sodium oxide?

    actually, i think you’re right that there’s no reaction. but since it’s a 5m question, i expect they want to to explain why there isn’t a reaction.

    for example, sodium is more higher in the reactivity series, it has a greater tendency to form its positive ions. Iron has less tendency to form iron(II) ions, hence cannot displace sodium from the oxide.

    btw. is it correct, that sodium oxide is a very stable compound, as there’s a stronger attraction of the nucleus on the electron(as valance electron quite near to nucleus)

    please do point out my mistakes if i have any misconceptions.

  5. hey.. just checking if these are correct.

    Q20 D

    cos i don’t think the indicators actually react with one another. and at pH5, both methyl orange & bromothymol blue are yellow, and phenolphthalein is colourless.

    Q21 B

    chlorine displace iodide ions.
    ionic eqn: Cl2 + 2I- -> I2 + Cl2
    so the solution will turn from yellowish-green to reddish brown.

    Q22 B

    cos MgCO3 is insoluble. other compounds such as KBr, NH4CO3 & K2SO4 are soluble.

    -lijun(:

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