POK OI HOSPITAL TANG PUI KING MEMORIAL COLLEGE

POK OI HOSPITAL TANG PUI KING MEMORIAL COLLEGE

FIRST TERM TEST ( 1997-1998 )

S6 CHEMISTRY

Class : 6 S Date : __________________

Name : ________________ Time allowed: 90 minutes

Class Number : ________________ ( P.1 - P.4 /38 )

Instructions to students:

1. Answer ALL questions.

2. Write your answers on the foolscap papers provided.

3. The total score is 100 marks.

Section A ( 50% )

1. What are the characteristics of a hydrogen spectrum?

( 8 marks )

2. Give the ground state electronic configuration of the following species:

(a) C

(b) K

(d) Cl^-

( 8 marks )

3. 10 cm³ of a hydrocarbon, C₄H_x were allowed to react with an excess of oxygen at

150 °C and 1 atmosphere. There was an expansion of 10 cm³. Deduce the value of x.

( 7 marks )

4. Potassium iodate(V) is a primary standard. It reacts with iodide ions in the presence of

acid to form iodine:

IO₃^- (aq) + 5I ^- (aq) + 6H⁺ (aq) ® 3I₂ (aq) + 3H₂O (l)

1.015g of potassium iodate(V) are dissolved and made up to 250 cm³. To a 25.0 cm³

portion are added an excess of potassium iodide and dilute sulphuric acid. The solution

is titrated with a solution of sodium thiosulphate, starch solution being added near the

end- point. 29.8 cm³ of thiosulphate solution are required. Calculate the concentration

of the thiosulphate solution.

( 7 marks )

5. Calculate the total vapour pressure of a solution containing 50.0g heptane, C₇H₁₆ and

38.0g octane, C₈H₁₈ at 20 °C. The vapour pressures of the pure heptane and octane at

20 °C are 473 Pa and 140 Pa respectively.

( 7 marks )

6. A current of 1.00A flowing for 1 hour 50 minutes deposits 2.15g of copper from an

aqueous solution of copper(II) sulphate. If the Avogadro constant is 6.02 x 10²³ mol^-1,

calculate the charge on an electron. ( 6 marks )

7. 0.25 cm³ of a volatile liquid X, of density 1.3 g cm^-3, were injected into a gas syringe

which was maintained at 200 °C. 100 cm³ of vapour were formed. If the pressure of

the atmosphere is 757 mmHg, calculate the relative molecular mass of X.

[ universal gas constant, R = 0.082 dm³ atm K^-1 mol^-1 ] ( 7 marks )

Section B ( 50 % )

8. Borax ( Na₂B₄O₇. 10H₂O ) has a relative formula mass of 381.37. It is often used as a

primary standard in acid- base titrations.

You are provided with a pure sample of borax and distilled water and the usual

laboratory apparatus and instruments.

(a) What is meant by a “ primary standard”? ( 2 marks )

(b) Calculate the mass of borax required in the preparation of 0.1 M standard borax

solution. ( 2 marks )

are necessary. ( 11 marks )

9. (a) In 1909, Geiger reported the results of experiments in which a beam of a- particles

was directed at very thin metallic foils. The large majority of the a- particles went

through the foils without deflection, but some were deflected or scattered

backwards.

(I) What is an a- particle? State its mass number and its charge.

(II) Why were most a- particles not deflected?

(III) Why were some a- particles deflected or scattered backwards?

(IV) What do these experiments illustrate about the structure of the metal atoms?

(V) State one other method of deflecting a- particles.

(VI) Ra- 226 was formed from emitting an a- particle. Give the equation of the

nuclear reaction involved. ( 9 marks )

(b) (I) Define the relative atomic mass of an element. Why do values of relative

atomic mass have no units?

(II) The figure below shows a mass spectrum of chlorine.

Identify the peaks in the mass spectrum. Hence calculate the relative atomic

mass of chlorine.

(III) Oxygen sample obtained from the Artic has a relative atomic mass of 15.994

whereas another sample from the Antartica has a relative atomic mass of

16.000. Why is there a difference in these values? ( 9 marks )

10. (a) Figure below is a graph of the first ionization energies against atomic number for

the elements with atomic numbers between 3 and 20.

(I) Explain the general increase in ionization energy between A and D and

between E and F.

(II) Give a reason to explain the dips in the graph at points B and C.

(III) Explain the large decrease from D to E. ( 8 marks )

(b) The first three ionization energies ( ionization potentials ) for calcium are:

1 st ionization energy : 510

2 nd ionization energy : 1150

3 rd ionization energy : 49400

All values are in kJ mol^-1.

(I) Show by means of an equation what is meant by the statement:

The third ionization on energy of calcium is 49400 kJ mol^-1.

(II) Briefly explain the increase in value of these three ionization energies for

calcium. ( 5 marks )

and II of the Periodic Table are shown below:

Na⁺ : 0.095 K⁺ : 0.133 Rb⁺ : 0.148

Mg²⁺ : 0.065 Ca²⁺ : 0.099 Sr²⁺ : 0.113

Comment upon the trends in the values. ( 4 marks )

END OF PAPER