welcome to mubula resource center of excellence: 1997 uace chemistry

Wednesday, 14 September 2016

1997 uace chemistry

PAPER 1

SECTION A

Answer all questions from this section.

1. Explain the following observations.

When ammonia solution is added to a solution containing aluminium ions, a white precipitate is formed. In the presence of ammonia chloride aluminium ions do not form a precipitate with ammonia solution

……………………………………………………………………………………………………………………………………………………………………………………………………

2. (a)The pH of a 0.10Msolution of a weak acid HX is 3.30.

Calculate the acid dissociation constant of HX

(b)When 50ml of 0.10M solution of ammonium hydroxide solution was added to 50ml of 0.10M solution of hydrochloric acid, the resulting solution had a pH less than 7.explain the observation

3. The diagram below shows the e.m.f cell for the reaction between iron(III)chloride and potassium iodide solution.

(a)Indicate on the diagram the direction of flow of current.

(b)State what is observed at the

(i)anode

(ii)cathode

4. 5.5g of non volatile substance B was dissolved in 125g of a solvent. The vapour pressure curves for the solution and pure solvent at constant pressure P are shown below.

(a)Identify the curve for the solution

(b)Calculate the molecular mass of B

(The boiling point elevation constant for the solvent, K=0.52⁰C per mole^-1kg^-1)

5. Ethanol (bp 78.5⁰C) and tetrachlromethane (bp 76.8⁰C) form an azeotropic mixture of boiling point 65.0⁰C.

(i)What is meant by azeotropic mixture

(ii)Draw a boiling point diagram for the ethanol-tetrachlromethane mixture

(iii)Explain why ethanol and tetrachlromethane form an azeotropic mixture

6. (a)Write the expression for the distribution of ammonia between water and tetrachlromethane.

(b) 100cm³ of a solution containing 0.171g of ammonia in trichloromethane was shaken with 10cm³of water until equilibrium was attained at room temperature. Calculate the number of ammonia in the trichloromethane layer.(the distribution coefficient of ammonia between water and trichloromethane at room temperature is 27.5)

7. A solution containing 50gof ethane-1,2-diol and 30.0g of water.

(a)Calculate the boiling point of the solution

(H=1,O=16,C=12,Kb(water)=0.52K mol^-1 kg^-1)

(b)State any assumption you made in the calculation

8. Complete each of the following equations

9. 2.8g of an alkane, C_nH_2n.reacted completely with 8.0g of bromine

(a)Calculate the number of moles of bromine molecules that reacted with alkene

(ii)Deduce the number of moles of the alkene that reacted

(1 mole of bromine molecules reacts with 1 mole of alkene)

(b) (i)Calculate the formula mass of alkene

(ii)Determine the molecular formula of the alkene

SECTION B

Answer six questions from this part

10. The vapour pressures of water and an immiscible liquid X at different temperatures are given below.

(a) (i) Construct on fig 1 the vapour pressure curve for a mixture of water and X.

(ii)At what temperatures will the mixture boil at the following pressures.

(b) (i) After distilling at 101kPa pressure for some time the distillate was found to consist of 1.00g of water and 4.80g of X by using the graphs in the figure. Calculate the relative molecular mass of X

(ii)How will composition of the distillate change (if at all) during the distillation?

11. At 250C ammonia has a base ionisation constant,Kb,of,1.8x10^-5 mole dm^-3

(a)Write an expression for Kb of ammonia

(b)Calculate the concentration of OH^- in 0.1M ammonia solution at 25⁰C

State any assumptions made

(c) Calculate the change in OH^- concentration which occurs when 0.01 mole of ammonium chloride is added to 1.0dm³ of the solution in (b)

State any assumptions made

(d)Explain the change in OH^- concentration in (c).

12 (a) Write an equation for the hydrolysis of sodium ethanoate in water

(b)Write an expression for the hydrolysis constant, Kh of sodium ethanoate

(c)Calculate

(i)the value Kh for sodium ethanoate and indicate its units

(Ka for CH³COOH is 1.8x10^-5; Kw=1x10^-14)

(ii)the pH of a 0.1M sodium ethanoate solution

(d)State what would be the effect on the pH of the solution in ©(ii) if 1cm³ of 0.1M ethanoic acid was added to it

13. A gaseous hydrocarbon P consists of 92.3% carbon.

(a)Calculate the empirical formula of P

(b) 0.13g of P occupies 112cm³ at s.t.p

Calculate

(i)the formula mass of P

(ii)The molecular formula of P

14 (a) Write a general outer electronic configuration for elements in group11 of the periodic table.

(b)Describe the reaction if any between each of the following elements, beryllium, magnesium, calcium with

(i)Worm dilute sulphuric acid

(ii)Worm concentrated sodium hydroxide solution

15. Lead(II)sulphate is sparingly soluble in water.

(a)Write an expression for solubility product of lead (II)sulphate.

(b) 5g of lead(II) sulphate was shaken with 1dm³ of water

Determine the percentage of lead(II)sulphate that dissolved

(Ksp figures=1.6x10^-8,Pb=207,S=32,O=16)

(c) If 0.05M sulphuric acid was used instead of water in (b),calculate the percentage of lead(II)sulphate that dissolved and state any assumption you make

16. 0.9875g of an impure potassium manganate(VII)was dissolved in water to make 250cm³ of solution. When 20.0cm³ of this solution was acidified with dilute sulphuric acid, warmed and titrated against sodium ethanediote (oxalate)solution, made by dissolving 1.675g of anhydrous sodium ethanediote to make 250cm³ of solution,24.40cm³ of the sodium ethanediote solution was used(NaC₂O₄=134 and KMnO₄=158).

(a)Write an ionic equation for the reaction between sodium ethanediote and potassium manganate(VII).

(b)Determine the molar concentration of manganate(VII) ions

(d)Name one compound which is a common impurity in potassium manganate

17. When a pale green solid A was heated it decomposed to a green solid B and a gas that turned lime water milky was evolved. B dissolved in nitric acid to give a green solution C. When dilute sodium hydroxide solution was added to C a green precipitate D insoluble in excess alkali was formed C dissolved in aqueous ammonia to give a purplish blue solution.

(a)Identify compounds A, B and D and solution C

A………………………………………………………………………………………….

B…………………………………………………………………………………………

C……………………………………………………………………………………….

D……………………………………………………………………………………

(b)Write equation for the reaction between

(i) B and nitric acid

(ii) C and sodium hydroxide

(iii)D and ammonia

PAPER 2

SECTIONA

Answer three questions from this section

1. (a) A compound b contains 92.31% carbon and 7.69% hydrogen

Determine the empirical formula of B.

(b)B burns with a sooty flame and has a vapour density of 39

(i)Determine the molecular formula of B and write its molecular structure.

(c)Write equation and indicate a mechanism for the reaction between B and

(i) nitric acid

(ii) ethanoyl chloride

(iii) Propene

In each case indicate the other reagents necessary and the conditions for the reactions.

(d)Write equations to show how B can be synthesized.

2. (a) A solution contains a mixture of sodium hydroxide and sodium carbonate in 1 litre. Briefly describe how the percentage of sodium hydroxide in the mixture can be determined using a standard hydrochloric acid.

(b) Sketch a graph to show the variation of pH during the titration of hydrochloric acid with

(i)Sodium hydroxide

(ii)ammonia solution

Explain the shape of the graph in each case.

(c)Calculate the pH of the resultant solution when 10cm³ of 0.1M sodium hydroxide was added to 25cm³ of 0.1M ethanoic acid.(Ka of ethanoic acid = 1.8x10^-5)

3. Discuss the chemistry of zinc and iron showing

(a)similarities

(b)differences

Illustrate your answers with equations.

(your answer in (a) should include reactions with water, acids and non metals)

4. (a)With the aid of a diagram describe how standard electrode potential of an e.m.f cell can be measured.

(b)The standard electrode potentials of copper and zinc are given below

Cu²⁺(aq) + 2e Cu(s) E⁰=+0.34V

Zn²⁺(aq) + 2e Zn(s) E⁰=-0.76V

Write the cell diagram for zinc/copper cell and calculate the e.m.f of the cell

(d)During the electrolysis of dilute sulphuric acid using platinum electrodes a current of 2 amperes was passed for 6.7 minutes at room temperature.

Calculate the volume of the gas evolved at

(i)the anode

(ii)the cathode.

(1Faraday = 96500 coulombs;1 mole of gas occupies 24 dm³ at room conditions).

(e)Name one application of

(i)electrolysis

(ii)standard electrode potential.

SECTION B

Answer two questions from this section.

5. (a) (i) State Rault`s law.

(ii)Draw the vapour pressure/composition curve for mixtures of methanol and cyclohexene and explain the shape of the curve with reference to Rault’s law.

(b) (i)Explain the process of steam distillation.

(diagram not required)

(ii)Name one compound that can be isolated by steam distillation.

(c) When a compound Y was steam distilled at standard atmospheric pressure the distillation temperature was 86⁰C.The vapour pressure of water at 86⁰C is 740mm Hg. The distillate contained 85% by mass of water. Calculate the relative molecular mass of Y.

6. (a) (i)Explain what is meant by the term distribution coefficient.

(ii)Describe how distribution coefficient of ethanoic acid between water and butan-1-ol can be determined

(b) When 100cm³ of aqueous solution containing 30gof ethanoic acid was shaken with 50cm³ of butan-1-ol, 12g of ethanoic acid remained in the aqueous later. Calculate the distribution coefficient of ethanoic acid between water and butan-1-ol.

(c) The aqueous solution of ethanoic acid in (b) was shaken twice with 25cm3 portions of butan-1-ol.calculate the mass of ethanoic acid extracted.

(d) State two applications of the partition law.

7. (a) Describe briefly how chlorine is produced on a large scale.

(b)Describe briefly how chlorine can be converted to potassium chlorate(V) crystals in the laboratory.

(c) 2.0g of a mixture of potassium chloride and potassium chlorate were dissolved in 250cm3 of water.10.0cm3 of the solution was mixed with excess potassium iodide. The iodide liberated required 8.00cm3 of a 0.2M sodium thiosulphate solution for complete reaction potassium chlorate and potassium iodide react according to the equation.

ClO₃^-(aq) + 6H⁺(aq) + 6I^-(aq) 3I₂(aq) + Cl^-(aq) + 3H₂O(l).

Calculate the percentage of potassium chlorate in the mixture.

(d) Explain why chlorine is more soluble in dilute sodium hydroxide than in water.

8. Write equation to show how the following compounds can be synthesized

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Wednesday, 14 September 2016

1997 uace chemistry

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