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86

Credit will be given for strict adherence to instructions, for observations precisely recorded and for accurate inferences. All tests observations and inferences must be clearly entered in your answer book. in ink, at the time they are made.

 H is a mixture of an element and an organic compound. Carry out the following exercises on H. Record your observations and identify any gases evolved. State the conclusion you draw from the result of each test

(a) Put all of H in a beaker and add about 10cm\(^3\) of distilled water. Stir the mixture thoroughly and filter  Keep both the filtrate and the residue. Test the filtrate with litmus paper.

(b) Divide the filtrate into two portions

(i) To the first portion add two to three drops of acidified potassium tetraoxomanganate (VII) and warm

(ii) To the second portion. add a few drops of a saturated solution of sodium hydrogentrioxocarbonate (IV)

(c)(i) Put all the residue in a test tube and add 5cm\(^3\) of dilute hydrochloric acid

(ii) To the resulting solution from (C)(i) above add aqueous sodium hydroxide in drops until it is in excess

(iii) From your inferences in (c)(i) and (c)(i) state what would be observed if 5cm\(^3\) of dilute trioxonitrate (V) acid were added to a portion of the residue and the mixture was warmed. 

View Answer & Discuss WAEC 1992
87

All your burette readings (initial and final), as well as the size of your pipette, must be recorded but on no account of experimental procedure is required. All calculations must be done in your answer book.

F is a solution, O a dibasic acid H\(_2\)X. G is a solution containing 1.00g of sodium hydroxide in 250cm\(^3\) of solution.

(a) Put F in the burette and titrate with 20cm\(^3\) or 25cm\(^3\) portion of G using methyl orange as indicator. Record the volume of your pipette. Tabulate your burette readings and calculate the average volume of F used 

(b) From your results and the information provided, calculate the;

(i) concentration of G in mol. dm\(^3\)

(ii) concentration of F in mol. dm\(^{-3}\)

(iii) molar mass of the acid H\(_2\)X, given that 100cm\(^3\) of solution F contained 0.4850 the acid.

The equation for the reaction is H\(_2\)X\(_{(aq)}\) + 2NaOH\(_{(aq)}\) \(\to\) Na\(_2\)X\(_{(aq)}\) + 2H\(_2\)O [H = 1; O = 16; Na = 23] 

 

 

View Answer & Discuss WAEC 1992
88

(a) Name the apparatus used in the laboratory to (i) Convert vapour into liquid during distillation

(ii) determine the volumetric composition of water

(ii) produce an intermittent supply of any gas which can be evolved by the action of a liquid on a solid without heating.

(b) Give the reason for the following;

(i) the edge of the lid of a desiccator should be greased

(ii) a standard solution of sodium hydroxide is not prepared by weighing out accurately a given mass of the solid and making it up to the required volume.

iii) after the reduction of copper (II) oxide by a a stream of hydrogen gas passed overs the heated oxide in a combustion tube, it is necessary to continue passing the gas over the residue until the residue cools.

(c) An acid-base indicator turns orange in an acidic solution, green in a neutral solution and blue in an alkaline solution. The table shows the results obtained when a few drops of the indicator were added to samples of some liquids.

Liquid  Colour of the indicator in the liquid 

Glucose solution 

Rainwater 

Lime water 

Vinegar

Stomach powder solution 

Green

Pale orange 

Blue 

Orange 

Pale blue 

 

(i) Arrange the liquids in increasing order of their expected pH values.

(ii) Explain why the colour of the indicator in the sample of rainwater is pale orange whereas in pure water it is green.

(iii) A given crystalline solid is suspected to be either sodium chloride or ammonium tetraoxosulphate (VI). Describe how you would use the indicator to identify the solid.

(iv) State with reason whether or not phenolphthalein indicator can be used to distinguish between the samples of glucose solution and vinegar.

(v) What would be the colour of methyl orange indicator in the lime water sample? 

 

 

View Answer & Discuss WAEC 1991
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89

A is a solution containing 0.50 mole of barium chloride per dm\(^3\). Solution B contains 1.0 mole of tricxocarbonate (IV) salt per dm\(^3\)

(a) State what would be observed and give the confirmatory test for any gases evolved if the following tests were performed

(i) mixing 2cm\(^3\) each of solutions A and B in a test tube

(ii) adding excess dilute hydrochloric acid to the mixture from (a)(i) above

 

(b) 10cm\(^3\) of solution A were measured into each of seven boiling tubes of uniform bore and various quantities of solution B were added respectively to the boiling tubes, The tubes were immersed in hot water. After the reaction, the height of the product in each of the tubes was measured. The results were as tabulated below.

Test tube  I II III IV V VI VII
Volume of sodium B added (cm\(^3\)) 2.0 3.0 4.0 5.0 6.0 7.0 8.0
Height of the product (to the nearest m) 12 18 24 30 30 30 30

 

(i) Plot a graph of the height of the product against the volume of solution B added to each test tube. Explain the shape of the graph

(ii) Calculate the amount (in mole) of the trioxocarbonate (IV) salt contained in the volume of solution B added to tube lV

(iii) Calculate the amount (in mole) of barium chloride contained in 10cm\(^3\) of solution A. Hence, determine the mole ratio of barium chloride and the trioxocarbonate (IV) Salt in the reaction

View Answer & Discuss WAEC 1991
90

C is a mixture of an organic and an inorganic salt. The test recorded in the table below were performed as indicated, 

(a)Copy and complete the table as appropriate. 

Test  Observation Inference
(i) C + water mixed thoroughly and filtered Partially soluble, colourless filtrate, white residue  

(ii) Portion of filtrate from (i) + NH\(_{3(aq)}\) in drops, then in excess 

White precipitate which dissolves in excess to give a colourless solution   
(iii) Portion of filtrate from (i) + NH\(_3\) in drops, then in excess White precipitate insoluble in excess   
(iv) Portion of filtrate from (i) + conc. HCl + heat  White precipitate which dissolves on warming and reappears on cooling   
(v) Portion of filtrate from (i) + conc. H\(_2\)SO\(_4\) + gentle warming Brown fumes produced on warming   
(vi) Residue from (i) + iodine solution  Blue-black colouration    
(vii) Residue from (i) + Fehling's solution + heat     
(vii) Residue from (i) + dil HCl  solution + heat + Fehling's solution     

 

(b) From the information provided and your inference, identify the compounds of C 

(c) Describe the action of heat on a solid sample of the inorganic component  Write an equation for the reaction.

(d) State what would be observed if an aqueous suspension of the organic component were heated to boiling 

View Answer & Discuss WAEC 1991
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