Pok Oi Hospital Tang Pui King Memorial College

1st Term Test ( 1996-1997 )

F.6    Chemistry

Class                      :               ________________                           Date        :               __________________

Name                      :               ________________                           Time allowed: 90 mins

Class No.               :               ________________                           ( P.1 - P.7/38 )

Instructions to students:

The total score of this paper is 100 marks.

Answer Section A in this question paper.

Answer Section B in the foolscap papers provided.

Section A (40%)

1. The diagram below shows the apparatus a student sets up to carry out an acid-base

    titration:

    (a) Why is a conical flask used for titration, not an ordinary beaker?

    (b) The student has committed several errors in setting up the apparatus. What

          adjustments must be made in order to obtain accurate results?           (4 marks)

2. Write down the electronic configuration of the following species:           (4 marks)

    (a) Cu

    (b) Se

    (c) Fe³⁺

    (d) Cl^-

3. Identify A to F in the following nuclear reactions:                                     (3 marks)

    (a)

    (b)

    (c)

    (d)

    (e)

    (f)

4. An enclosed vessel contains 2.8g of nitrogen and 14.2g of chlorine at 101.3 kPa and

    0 ^oC. Calculate the partial pressure of nitrogen if the temperature in the vessel is raised to 180 ^oC.

                                                                                                                          (4 marks)

5. When a sample of potassium chlorate was heated,

                            2 KClO₃ ® 2 KCl + 3O₂

    a volume of 550 cm³ of dry oxygen was collected at 21 ^oC and 0.95 atm.

    (a) Calculate the number of moles of oxygen collected.

    (b) Calculate the mass of oxygen collected.                           (4 marks)

    [R = 0.08206 dm³ atm mol^-1 K^-1]

6. A current of 0.750 A passes through 250 cm³ solution of 0.25 M copper(II) sulphate

    solution. How long will it take to deposit all the copper on the cathode?

    [ Faraday constant = 96500 C mol^-1]                                  (4 marks)

7. 10 cm³ of gaseous hydrocarbon were exploded with 70 cm³ of oxygen (in excess) in the presence of concentrated sulphuric acid. The volume of the residual gas was

     50 cm³. This was reduced to 20 cm³ on adding aqueous potassium hydroxide.

Determine the molecular formula of the hydrocarbon. All volumes were measured under room temperature and pressure.                                                                                           (6 marks)

8. Calculate the volume of carbon dioxide evolved (at room temperature and pressure)

    when 5.3g of anhydrous sodium carbonate reacts with 40 cm³ of 2 M hydrochloric

    acid.

                        Na₂CO₃(s) + 2 HCl(aq) ® 2 NaCl(aq) + CO₂(g) + H₂O(l)

         [Molar volume of a gas at r.t.p. = 24 dm³]                         (5 marks)

9. 9.50g of hydrated copper(II) sulphate CuSO₄.nH₂O is strongly heated. When all the

    water of crystallization is driven off, the anhydrous compound left is found to weigh

    6.08g. What is the molecular formula of the hydrated compound?       (6 marks)

END OF SECTION A

Section B (60%)

1. The following apparatus is an improved version over the one originally designed by

    Meyer. The volatile liquid X is stored in the hypodermic syringe and is injected into

    the graduated syringe which is heated up with steam.

    The followings are some typical results for the liquid X:

                 Mass of liquid X = 0.355g

                 Initial volume of air in the graduated syringe = 11.0 cm³

                 Final volume of air + vapour of X in the graduated syringe = 83.0 cm³

                 Temperature of steam = 99^oC

                 Atmosphere pressure = 733 mmHg

    (a) How can we determine the mass of volatile liquid X?

    (b) If there is a small amount of air inside the hypodermic syringe, will this lead to a

         serious error? Explain.

    (c) Is it necessary to keep the temperature of steam used at 100 ^oC?

    (d) Determine the molecular mass of the liquid X.

         [R = 0.08206 dm³ atm mol^-1 K^-1]

    (e) The molecular mass of X obtained will not be very accurate. Can you suggest

         three possible sources of error in this experiment?               (10 marks)

2. (a) Define the relative atomic mass of an element.

    (b) (I) The following figure is a mass spectrometer trace for an organic compound

              investigated in a mass spectrometer.

        Suggest a value for the relative molecular mass of this compound and identify three elements which it

        might contain.

         (II) Chlorine in nature consists of two isotopes of mass numbers 35 and 37 respectively, having a relative abundance of 3 : 1. If a sample of tetrachloromethane is introduced into a mass spectrometer, how many peaks will occur in the CCl₃⁺ region of the spectrum?                     (7 marks)

3. 1.00g of hydrated ethanedioic acid, H₂C₂O₄.2H₂O was dissolved in deionized water

    and made up to 250.0 cm³ of a standard solution. A 25.0 cm³ of the resulting standard solution was titrated with an acidified potassium manganate(VII) solution, 19.8 cm³ of the acidified potassium manganate(VII) was required for a complete reaction.

    (a) What is a ‘standard solution’? Calculate the molar concentration of the prepared

         standard solution, H₂C₂O₄(aq).

    (b) The reaction between ethanedioic acid solution and acidified potassium

         manganate(VII) solution can be represented by the following equation:

         5 H₂C₂O₄(aq) + 2 MnO₄^-(aq) + 6 H⁺(aq) ® 2 Mn²⁺ (aq) + 10 CO₂(g) + 8 H₂O(l)

         (I)  Is it necessary to add any indicator to this titration? Explain your answer briefly.

         (II) Calculate the molar concentration of the acidified potassium manganate(VII) solution.

    (c) If 25.0 cm³ of an iron(II) sulphate solution was titrated against the above acidified potassium manganate(VII) solution, 17.2 cm³ of the latter was required for complete reaction. Calculate the concentration of iron(II) sulphate solution.

     Given:  5 Fe²⁺ (aq) + MnO₄^-(aq) + 8 H⁺ (aq) ® 5 Fe³⁺ (aq) + Mn²⁺ (aq) + 4 H₂O(l)

    (d) During the titration process between iron(II) sulphate and acidified potassium manganate(VII) solution. If the following glasswares contain a little deionized water,

         (I)    a conical flask,

         (II)   a pipette, and

         (III)  a burette.

      How will the accuracy of the above titration be affected? Explain your answer briefly.                                                     (14 marks)

4. (a) Define the first ionization enthalpy of an atom.

    (b) Which element has the highest first ionization enthalpy of all elements? 

         Briefly explain your answer.

    (c) The first ionization enthalpies of elements from sodium to argon are given 

       below:

          1. Why does the first ionization enthalpy generally increase from sodium to argon?

2. Why is the first ionization enthalpy of aluminium less than magnesium?

3. Why is the first ionization enthalpy of sulphur less than phosphorus?

    (d) The following species are isoelectronic:

                        K⁺, Cl^-, Ar, Ca²⁺

         When one electron is to be removed from these species, which one requires the

         highest ionization enthalpy? Briefly explain your answer.          (14 marks)

5. (a) How does the line emission spectrum of hydrogen support the view that the

         extranuclear electrons in an atom occupy different energy level?

  (b) The following figure shows the atomic emission spectrum of hydrogen.

         (I)    Why is the spectrum composed of discrete lines rather than being continuous?

(II)   What does each line represent?

(III)  Explain why the lines become progressively closer to each other.       

    (c) The frequency,n, of the different lines in the Lyman series of the hydrogen

         spectrum is given by the expression:

       where c is the velocity of light, R_H is a constant and n is an integer greater than 1.

      The frequency of the first line in the Lyman series is 2.47 x 10¹⁵ Hz. Use this

      value, together with the values of the relevant physical constants to calculate the

      ionization energy of the hydrogen.

[Planck constant, h = 6.63 x 10^-34 Js; Velocity of light, c = 3.00 x 10⁸ ms^-1;

Avogardro constant, N = 6.023 x 10²³ mol^-1]                        (15 marks)

END OF SECTION B

END OF PAPER