Possible WAEC 2023 Chemistry Practical Salt Analysis

 

There are two samples of compounds to be examined in this coming examination: zinc oxide and glucose. Glucose is tagged C_n and zinc oxide is tagged D_n.

   Glucose is an example of a monosaccharide. It is a strong reducing agent because of the presence of the –CHO group. It is dehydrated to carbon by concentrated tetraoxosulphate (VI) acid like all other carbohydrates.

  Zinc oxide is a white insoluble powder which turns yellow when it is hot. As an amphoteric oxide, it can dissolve in both acids and alkalis.

Analysis of Glucose

The reducing property of glucose can be identified by the following experiments:

·         Its reaction with acidified potassium tetraoxomanganate (VII) solution. Glucose decolorizes acidified potassium tetraoxomanganate (VII) solution from purple to colourless.

·         Its reaction with acidified potassium heptaoxodichromate (VI) solution. Glucose turns acidified potassium heptaoxodichromate (VI) solution from orange to green.

·         It also reduces ammoniacal silver trioxonitrate (V) (Tollen’s reagent) to metallic silver. This is one of the confirmatory tests for glucose

·         Glucose forms brick red precipitate with Fehling’s solution which is another confirmatory test for glucose

·         With Benedict’s solution, glucose forms a yellow precipitate. This test can also be used to confirm the presence of glucose

Analysis of zinc oxide

The common reagents for the test are sodium hydroxide and aqueous ammonia

Test with sodium hydroxide

Add a few drops of dilute sodium hydroxide to a solution of the test sample. The presence of zinc ions can be identified by the formation of a white gelatinous precipitate which is soluble in excess sodium hydroxide. However, lead ions and aluminium ions give similar results.

Test with Aqueous ammonia

Add some drops of aqueous ammonia to a solution of the unknown sample. The formation of a white gelatinous precipitate which is soluble in excess aqueous ammonia confirms the presence of Zinc ions. Lead ions and aluminium ions form a precipitate which is insoluble in excess.

Example Question

C is a sample of an organic compound and D is an inorganic compound. Carry out the tests below, and record your observation and inference.

 

s/n	Test	Observation	Inference
1 (a)	C + distilled water. Divide into two portions	Soluble and gives a clear solution
(b)	To the first portion from 1(a) above, add acidified K2Cr2O7 solution.		C is a reducing agent. Reducing sugar (glucose) is likely present
(c)	A brick	A brick-red precipitate is formed
2 (I)	D + distilled water		Insoluble ions of Ca, Al, Pb, and Zn are likely to present
(II)	D + dilute HCl, and divide into two portions	Soluble
(III)	To the first portion from 2(II) above, add dilute NaOH solution in drops then in excess		Ions like Pb, Al or Zn are likely to present.
(IV)	To the second portion from 2(II) above, add aqueous ammonia	The white gelatinous precipitate, soluble in excess

 

 

 

 

Answers

s/n	Test	Observation	Inference
1 (a)	C + distilled water. Divide into two portions	Soluble and gives a clear solution	Soluble organic compounds like reducing sugar (glucose), organic acids likely present
(b)	To the first portion from 1(a) above, add acidified K2Cr2O7 solution.	It turns from orange to green	C is a reducing agent. Reducing sugar (glucose) is likely present
(c)	To the second portion from 1(a) above, add Fehling’s solution	A brick-red precipitate is formed	Reducing sugar (glucose) confirmed
2 (I)	D + distilled water	White insoluble compound	Insoluble ions of Ca, Al, Pb, and Zn are likely to present
(II)	D + dilute HCl, and divide into two portions	soluble	Soluble ions of Ca, Al, and  Zn are likely to present
(II)	To the first portion from 2(II) above, add dilute NaOH solution in drops then in excess	The white gelatinous precipitate, soluble in excess NaOH	Ions like Pb, Al or Zn are likely to present.
(IV)	To the second portion from 2(II) above, add aqueous ammonia in drops then excess	The white gelatinous precipitate, soluble in excess aqueous ammonia	Zinc ion confirmed