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1998 WAEC Chemistry Practical Credit will be given for strict adherence to instructions, for observations precisely recorded, and for...

Chemistry
WAEC 1998

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, a the time they are made.

C is a mixture of two simple salts. Carry out the following exercises on C. Record your observations and identify any gases evolved. State the conclusion you draw from the result of each test.

(a) Add about 10cm\(^3\) of distilled water to all of C in a boiling tube. Shake the mixture and filter: Keep both the filtrate and the residue.

(b) Divide the filtrate into two portions.

(i) To the first portion, add about 2cm\(^3\) of barium chloride solution, followed by dilute hydrochloric acid in excess. Warm the mixture and identify any gas evolved.

(ii) To the second portion, add few drops of acidified potassium tetraoxomanganate (VIl) solution and shake.

(C) Put the residue in a test tube and add about 3cm\(^3\) of dilute hydrochloric acid Warm gently and identify any gas evolved. Keep the mixture.

(d) Divide the clear solution from (c) above into two portions

(i) To the first portion, add sodium hydroxide solution in drops and then in excess.

(ii) To the second, add aqueous ammonia in drops and then in excess.

 

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Explanation

C is a mixture of Na\(_2\)SO\(_3\)  and CuCO\(_3\)

 

Test Observation I Inference
(a) C  + water; filtered 

Colourless filtrate or solution 

Green residue

 
(b)(i) Filtrate form (a) + BaCl\(_{2(aq)}\) + Hcl and warm

White precipitate 

Effervescence gas evolved which is colourless which is colourless with pungent choking or sharp irritating smell which decolourise KMnO\(_4\). The ppt. dissolved

The gas can also decolourise K\(_2\)Cr\(_2\)O\(_7\) 

CO\(^{2-}_3\) or SO\(_3^{2-}\) or SO\(_4^{2-}\) or S\(^{2-}\) 

Gas is SO\(_2\) from SO\(_3^2\)

(ii) Filtrate from (a) + KMnO\(_{4(aq)}\) KMnO\(_4\), decolourised or the purple colour discharged  Reducing agent present 
(c) Residue + dil. HCl + heat  Gas evolved, effervescence colourless, odourless turns lime water milky blue/green solution Gas is CO\(_2\) from CO\(_3^{2-}\)/HCO\(_3\)
(d)(i) Solution from (c) + NaOH\(_{(aq)}\) in drops then in excess 

Blue ppt. 

ppt. remained insoluble 

Cu\(^{2+}\) present 

 

(ii) Solution from (c) + NH\(_{3(aq)}\) in drops, then in excess 

pale blue ppt.

ppt. dissolved to give a deep bless solution 

CU\(^{2+}\) 

 


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WAEC May/June 2024 - Practice for Objective & Theory - From 1988 till date, download app now - 99995
Post-UTME Past Questions - Original materials are available here - Download PDF for your school of choice + 1 year SMS alerts
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WAEC Past Questions, Objective & Theory, Study 100% offline, Download app now - 24709
WAEC Past Questions, Objective & Theory, Study 100% offline, Download app now - 24709
WAEC May/June 2024 - Practice for Objective & Theory - From 1988 till date, download app now - 99995
WAEC offline past questions - with all answers and explanations in one app - Download for free
Post-UTME Past Questions - Original materials are available here - Download PDF for your school of choice + 1 year SMS alerts