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Question 2
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Credit will be given for strict adherence to the 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.
C is a mixture of two inorganic salts. Carry out the following exercises on C. Record your observations and identify any gas(es) evolved. State the conclusion you draw from the result of each test.
(a) Put all of C in a test tube and add about 10 cm3 of distilled water.
Stir. Filter and keep the filtrate and the residue.
(b) Put the residue into a test tube and add about 5 cm3 of dilute HCl. Shake to dissolve.
(i) To about 2 cm3 of the solution, add NaOH(aq) in drops and then, in excess .
(ii) To another 2 cm3 portion of the solution, add NH3(aq) in drops and then in excess.
(c) To about 2 cm3 portion of the filtrate, add few drops of dilute HN03 and then AgN03(aq) followed by aqueous NH3 in excess. [17 marks]
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The question was attempted by most of the candidates and the performance was fair.
In (a), most candidates were able to separate the soluble salt from the insoluble salt but . some of them lost marks because they were unable to give the observable colour of the residue while a few others referred to the residue as "precipitate".
In (b), few candidates were unable to perform the confirmatory test on the colourless gas evolved on addition of dil HCI on the residue and shake. They were expected to pass the gas into lime water which turns milky for the correct inference C02 from CO32-or HC03 - to be made.
In (b)(i) and (ii), majority of the candidates carried out the test on salt C rather than on portions of the resulting solution in b. Hence, they lost some marks. Also, a good number of them were unable to correctly write the correct charge on ions e.g. Zn2+ instead Zn2+ of and A13+ instead of Al3+.
In (c), some candidates lost marks because they did not adhere to the instruction for observation to be made step-wisely. On addition of dil HN03 on the filtrate, no visible reaction would be observed until aqueous AgN03 was added and white precipitate was formed for chloride ion to be present.
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