Module 1 June 1996

Return to AS and A2 Chemistry

1. (a) Calculate the number of a and the number of b particles emitted in the change ²³²₉₀ Th to ²²⁴₈₈ Ra.
Briefly explain your answer.

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     (b) A sample of ²²⁴₈₈ Ra reduces to one-eighth of its original activity after 10.92 days. Calculate its half-life.

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     (c) Radioactive decay of ²²⁴₈₈ Ra results in ²²⁰₈₆ Rn.

(i) Write a nuclear equation for this reaction.

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(ii) ²²⁰₈₆ Rn is radioactive but unreactive chemically.

Explain this difference.

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(iii) Radon gas is given off naturally by some building materials. Given that ²²⁰₈₆ Rn is an a emitter,
comment on why this isotope is considered to be a health hazard.

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(5)
(d) From the position of radium in the Periodic table, predict the following:

(i) the formula of radium carbonate;
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(ii) the equation for the thermal decomposition of radium carbonate;
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(iii) how the decomposition temperature required in (d)(ii) would compare with that required for magnesium carbonate.
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(e) If the gas from (d)(ii) is passed into dilute aqueous calcium hydroxide, a white precipitate forms which shows no radioactivity. Explain this observation.
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Total 16 marks

2. (a) In the table below, give the formulae of the chlorides of the elements of period 3, other than silicon.

Element	Na	Mg	Al	Si	P	S
Formula of chloride				SiCl₄

(2)
(b) Calculate the percentage by mass of silicon in silicon tetrachloride.

(2)
(c) (i) Draw a dot and cross diagram to show the bonding in silicon tetrachloride.

(2)
(ii) Draw the shape of this molecule.

Explain your answer in terms of the Electron Pair Repulsion Theory.
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(iii) State the shape of a molecule of AlCl₃and explain why it is different from that of SiCl₄.
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(d) (i) Give an equation for the reaction of SiCl₄ with cold water.
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(ii) How does the behaviour of carbon tetrachloride with cold water compare with this? Explain any differences.
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Total 14 marks

3. (a) Give the electronic configuration of the following ions.

(b) Outline, giving the reagent, the essential conditions and the equation, how a sample of anhydrous iron(II) chloride could be made from iron.
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(c) Give the formula and name of the metal-containing ion formed when iron(II) chloride is dissolved in water. State the shape of this ion.
Formula:   ..........................................................................................................................................................
Name:      ..........................................................................................................................................................
Shape:     ................................................................................................................................................... (3)

(d) (i) Describe what you would observe if dilute sodium hydroxide is added to the solution in (c).
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(ii) Give an equation for this reaction and state the type of reaction occurring.
Equation: ......................................................................................................................................................
Type of reaction: ........................................................................................................................................ (4)

(e) (i) What further change would you observe if the product from (d)(i) was allowed to stand in air?
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(ii) What type of reaction is occurring in the change?
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(iii) Explain briefly, in the light of your answer to (a), why you might expect this reaction to occur.
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Total 16 marks

4. A solution of a weak acid H₂X was made by dissolving 2.25g of solid H₂X in water to give 500cm³ of solution. On titration, 25.0cm³ of this solution was completely neutralised by 25.0 cm³of sodium hydroxide solution containing 0.100 moldm^-3.
(a) Write an equation for the reaction.
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(b) (i) Calculate the number of moles of NaOH in 25.0cm³ of 0.100 moldm^-3 solution.
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(ii) How many moles of H₂X would be required to react with this quantity of NaOH?
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(iii) Calculate the relative molecular mass of H₂X.

(iv) A hydrated form of the acid also exists, H₂X.yH₂O. A solution containing 6.30 gdm-3 of the hydrated acid has the same (molar) concentration as the solution of the anhydrous acid, H₂X, originally used. Using this information and your answer from (b), calculate the value of y.

(8)
(c) The presence of sulphur dioxide in the atmosphere is the main cause of acid rain. Outline a method which could be used to estimate quantitatively the concentration of sulphur dioxide in a sample of air.
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Total 14 marks