1989 WAEC Chemistry Practical Credit will be given for strict adherence to instructions, for observations precisely recorded, and for accurate...

WAEC 1989

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.

Carry out the following exercises on sample F. Record your observations and identify any gases evolved. State the conclusion you draw from the result of each test.

(a) Put all of F into a beaker and add about 10cm\(^3) of water. Stir the mixture and filter. Keep both the residue and the filtrate.

 

(b)(i) Test the filtrate with litmus paper.

(ii) Add a few drops of the fehling's solution provided to about 2cm\(^3\) of the filtrate in a test tube. Boil the mixture.

 

(c) Put the residue from (a) above into a test tube and add about 5cm\(3\) of hydrochloric acid. Identify the gas evolved.

(d) To about 2cm\(^3\) of the clear solution from (c) above add dilute sodium into hydroxide solution dropwise and then in excess.

(e) From your results deduce what F is and its constituent

Explanation
  Test Observation Inference
(a)  F + water and filtered  The residue is black and filtrate is colourless   

(b)(i)

(ii)

Filtrate + litmus paper 

Filtrate + fehlings solution 

No effect 

Brick - red ppt. 

 

Neutral solution 

(c) Residue + dil. HCl A colourless, odourless gas evolved. The gas gives a pop sound with a lighted splint, light green solution formed

The gas is hydrogen 

The residue is a metal above hydrogen in the activity series. Fe\(^{2+}\) suspected 

(d) Clear solution + NaOH\(_{aq}\) Dirty green ppt., insoluble in excess NaOH\(_{(aq)}\)  Fe\(^{2+}\) confirmed 

 

(e) From the result above, F is confirmed to be a mixture. The residue is grey and metal, that is iron filings. The other component is soluble in water and reduces fehling's solution, which is either glucose or fructose.  


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