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1990 WAEC Chemistry Practical Credit will be given for strict adherence to the instructions, for observations precisely recorded, and for accurate inferences....

Chemistry
WAEC 1990

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. Carry out the following exercises on sample C.

Record your observations and identify any gases evolved 

 

(a) Put all of C into a test tube and add about 5cm\(^3\) of water. Shake the mixture and filter. Keep both the residue and the filtrate

 

(b) Divide the filtrate into two portions;

(i)  To the first portion, add a few drops of dilute trioxonitrate (V) acid followed by about 1cm\(^3\) of silver trioxonitrate (V) solution 

(ii) Add excess aqueous ammonia to the mixture in (i) above

(iii) To the second portion, add about 2cm\(^3\) of dilute sodium hydroxide solution and warm gently

 

(c) Add about 2cm\(^3\) of dilute hydrochloric acid to the residue from (a) above and warm gently. Filter if necessary and divide the resulting solutio into two portions.

(i) To the first portion add dilute sodium hydroxide solution in drops until it is in excess

(ii) To the second portion, add aqueus ammonia in drops until it is in excess.

 

 

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Explanation
Test Observation Inference 

(a)(i) C + water and filtered 

part of sample dissolves 

colourless filtrate salt

C is a mixtue of soluble salt

(b)(i) Filtrate + dil. HNO\(_3\) + ANO\(_3\) 

(ii) The mixture from (b)(i) above + excess aqeuous NH\(_3\)  

White ppt.

ppt. dissolves 

Cl present 

Cl confirmed

(iii) Filtrte + NaOH\(_{(aq)}\) and warmed  colourless gas evolved with characteristic choking smell; turns limit blue and forms white fumes with conc. HCl acid stopper/vapour 

Gas is NH\(_3\)

NH\(_4^+\) present

(c)Residue from (a) + dil. HCl Residue dissolves liberating a colourless gas which turns lime water milky 

the gas is CO\(_2\)

CO\(_3^{2-}\) present 

(c)(i) Solution from (C) + NaOH\(_{(aq)}\) white ppt; soluble in excess NaOH\(_{(aq)}\)  Zn\(^{2+}\) or Pb\(^{2+}\) present 
(ii) Solution from C + NH\(_{3(aq)}\)  white ppt. solution in excess NH\(_{3(aq)}\)  Zn\(^{2+}\) confirmed 

 

From the results, C is a mixture of ZnCO\(_3\) and NH\(_4\)Cl


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JAMB CBT 2024 - Candidates, Schools, Centres, Resellers - Get Ready!
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WAEC May/June 2024 - Practice for Objective & Theory - From 1988 till date, download app now - 99995
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JAMB CBT 2024 - Candidates, Schools, Centres, Resellers - Get Ready!
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WAEC May/June 2024 - Practice for Objective & Theory - From 1988 till date, download app now - 99995
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