Question

2. C is an organic compound. D is an inorganic compound. Carry out the following exercises on C and D. Record your observations and identify any gas(es) evolved. State the conclusions you draw from the results of each test.
(a) Put C into a test tube and add about 5 cm3 of distilled water and shake well. Divide the resulting solution into two portions.
 (i) Test the first portion with litmus paper.
 (ii) To the second portion, add about 1 cm3 of Fehling's solutions A and B, and heat.
(b) Divide D into two portions.
 (i) Put the first portion into a dry boiling tube, heat, and then allow to cool.
 (ii) Add about 5 cm3 of dilute HCl to the second portion and heat; allow it to cool and filter if necessary. Divide the resulting solution into two portions.
  (iii) To the first portion from (b)(ii), add NaOH(aq) in drops and then in excess.
  (iv) To the second portion from (b)(ii), add aqueous NH3 in drops and then in excess.
(c) State what would be observed if a few drops of NaOH solution is added to 2 cm3 of solutions of each of the following salts:
 (i) Pb(NO3)2;
 (ii) Fe2(SO4)3.

Accepted Answer

C = Glucose. D = Zinc oxide (ZnO).

(a) C + distilled water → dissolves to form a colourless solution → C is a soluble compound.
(i) C(aq) + litmus paper → no effect on litmus paper (red/blue stays same) → solution of C is neutral.
(ii) C(aq) + Fehling's solution + heat → a brick-red precipitate is formed → C is a reducing sugar (a reducing agent is present).

(b)(i) D + heat, then cool → solid turns yellow when hot, and turns white again on cooling → ZnO is present (characteristic of zinc oxide).
(ii) D + dilute HCl + heat → dissolves to form a colourless solution → D is a basic oxide.
(iii) 1st portion + NaOH(aq) in drops → a white gelatinous precipitate forms (Zn²⁺ or Al³⁺ may be present); in excess → the precipitate dissolves (confirms Zn²⁺ or Al³⁺, both amphoteric).
(iv) 2nd portion + NH3(aq) in drops → a white gelatinous precipitate forms (Zn²⁺ or Al³⁺ may be present); in excess → the precipitate dissolves (confirms Zn²⁺ specifically, since Al(OH)3 does not dissolve in excess NH3 — this narrows it down to Zn²⁺ being present).

(c)(i) Pb(NO3)2 + NaOH(aq) → a white (chalky) precipitate is formed.
(ii) Fe2(SO4)3 + NaOH(aq) → a reddish-brown (gelatinous) precipitate is formed.

WAEC Chemistry 2023 Theory — Question 9

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