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Syllabus and sample Questions

Questions will be generated generally based on, but not restricted to the syllabus of GCE ordinary Levels.  The questions will not carry the same style as O levels examination.  Some content knowledge is essential, but the focus of the questions is on analytical, reasoning and creative skills.  The questions will also test student's ability to process the information in the syllabus and apply the chemistry knowledge learnt to everyday life.

We discourage special coaching/tuition tailored for the preparation of SJChO but encourage a vigorous approach to chemistry learning through rationale & critical thinking coupled with practical & creative skills.  As such the committee does not provide any past year examination questions for the purpose of coaching or training.  One way to prepare for the Olympiad is to fully understand the concepts in GCE O Level syllabus and conduct group discussion or sharing sessions on the applications of those concepts.  Extensive reading will be helpful.

Round 1: Sample questions

1

Tellurium has a lower atomic number than iodine, but it has a higher relative atomic mass than iodine.  This is because:

 

A.Tellurium has more electrons than iodine.

 

B.Tellurium has more isotopes than iodine

 

C.One of the isotopes of tellurium has more neutrons than one of the isotopes of iodine.

 

D.The main isotopes of tellurium have more nucleons than the main isotopes of iodine.

   

 

 

2

Phenolphthalein is a common indicator for acid-base titrations with composition by mass of 75.46% carbon, 4.43% hydrogen and 20.11% oxygen.What is its empirical formula?

 

A.C7H3O2

 

B.C10H7O2

 

C.C15H4O2

 

D.C5H2O3

Round 2: Sample question

Students will be expected to be familiar with both the tap and clip versions of burettes.

Question 1: To study the relative oxidising ability of 3 reagents.

Chemicals & apparatus:

You are provided with

  • Solid iron(II) salt, FeX
  • Aqueous solution containing 3.16 g dm–3 of potassium manganate(VII)
  • Aqueous solution of substance A (communal burettes)
  • Dilute sulphuric acid

Data:

Ar of K = 39.1, Mn = 54.9, O = 16.0

Concentration of potassium manganate(VII) = 3.16 g dm–3

Procedures:

Part 1

  1. 1.Weigh out accurately about 5.0 g of the iron(II) salt. Record your readings in an appropriate manner.
  2. 2.Dissolve the iron (II) salt in about 50 cm3 of the dilute sulphuric acid in a small beaker. Transfer the solution and washings into a 250 cm3 graduated flask. Make up to the mark with deionised water.
  3. 3.Pipette 25.0 cm3 of the Fe2+ solution you prepared into a conical flask and add about 15 cm3 of dilute sulfuric acid.
  4. 4.Titrate this acidified solution of Fe2+ with the potassium manganate(VII) solution provided. Repeat the titration as many times as you think is necessary. Record your results and any colour changes.

Part 2

  1. 1.Pipette 25.0 cm3 of the Fe2+ solution you prepared into a conical flask and add about 15 cm3 of dilute sulfuric acid. Then add 10.00 cm3 of the aqueous solution of substance A from the communal burette.
  2. 2.Titrate the resulting mixture with the potassium manganate(VII) solution. One titration will suffice.
  3. 3.Repeat the above two steps (bulleted) with two further 25.0 cm3 portions of the Fe2+ solution you prepared, each time adding about 15 cm3 of dilute sulphuric acid as well, but instead add 20.00 cm3 and 25.00 cm3 of aqueous A respectively before titrating with potassium manganate(VII). Record your results in an appropriate manner.

Analysis of Results (20 Marks)

Part 1

The balanced equation of the reaction between Fe2+ and MnO4 ‐ is given as follow:

5Fe2+(aq) + MnO4(aq) + 8H+(aq) →5Fe3+(aq) + Mn2+(aq) + 4H2O(l)

(i)                  Calculate the concentration of Fe2+ in 1 dm3 of the solution you prepared, expressed as mol dm–3 and hence g dm–3.

(ii)                 Using the mass of the iron(II) salt you weighed out and your answers above,calculate the percentage by mass of Fe2+ in the iron(II) salt, and the relative formula mass of the iron(II) salt.

Part 2

(i)      Plot a graph of volume of potassium manganate(VII) solution against volume of aqueous A added. (The graph consists of two straight lines that intersect at the horizontal axis.)

(ii)    Estimate from your graph the volume of aqueous A which will be needed to react with 25.0 cm3 of the Fe2+ solution you prepared.

(iii)   Using your answer in Part 2(ii) and given that 1 mole of substance A reacts with 2 moles of Fe2+, calculate the concentration of A in the given solution.

(iv)   What conclusion can you draw from your graph about the relative oxidising abilityof the 3 substances: MnO4, Fe2+ and A?

Answers (20 Marks)

Mass of bottle and iron(II) salt / g

8.51

Mass of bottle and residual solid / g

3.51

Mass of iron(II) salt used / g

5.00

Part 1

(i)      No of moles of KMnO4 needed for titration =

No of moles of Fe2+ present in 25.0 cm3 =

                               

Concentration of Fe2+ = 0.0760 x 55.8 = 4.24 g dm-3

(ii)    Mass of iron (II) salt weighed out = 5.00 g.

Mass of Iron (II) salt in 1 dm3 of solution

Relative formula mass of the given Fe(II) salt

Part 2

(i)

(ii)    13.60 cm3 is needed to react with Fe2+.

(iii)   No of moles of A in 13.60 cm3      

                                                                                                 

(iv)  Oxidising ability of MnO­4- > A > Fe2+

Credit: Dr Mahesh Uttamchandani, DSO for providing the sample questions.