Chemistry 3 Nov/Dec 2012

Chemistry Paper 3,Nov/Dec 2012

Questions:		1	2	3		Main

General Comments
Weakness/Remedies
Strength

Question 2

Question 2

Q was a mixture of two inorganic salts. The mixture was dissolved in water and filtered.

Tests were carried out on both the filtrate and residue.
Copy and complete the table below and state the ions present in Q.

	*Test*	*Observation*	*Inference*
(i)	Portion of filtrate + dil. HCl + BaCl2(aq)	No visible reaction	---------------------- ----------------------
(ii)	Portion of filtrate + dil. HNO3 + AgNO3(aq).	No visible reaction	---------------------- ----------------------
(iii)	Portion of filtrate + dil. NaOH(aq) in drops and then in excess.	White gelatinous precipitate soluble in excess dilute NaOH(aq)	---------------------- ---------------------- ----------------------
(iv)	Portion of residue + dil. HNO3.	Effervescene of a colourless, odourless gas	---------------------- ----------------------
(v)	Portion of residue + dil. HCl. Resulting mixture heated and later cooled.	----------------------------- ----------------------------- -----------------------------	Pb2+ present
(vi)	Portion of residue + KI(aq).	----------------------------- -----------------------------	---------------------- ----------------------

Copy and complete the following table showing the characteristic test for each class of organic compound indicated.

	Test	Observation	Inference
(i)	Compound + NaHCO3(aq)	---------------------- ----------------------	Alkanoic acid present
(ii)	Compound + Htr/K2Cr2O7((aq) + heat	---------------------- ----------------------	Alkanol present

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OBSERVATION

This question was based on both qualitative inorganic and organic analysis. Majority or the candidates answered this question but the performance was poor.
In (a), few candidates gave the correct inferences to the test/observations provided. Some candidates lost mark in a(i) for not inferring that SO32-, CO32- or SO42-1 was absent.

In (b) only a few candidates gave the required observations for the test and inferences provided.

The expected answers include:
2.
(a)

Test	Observation	Inference
Portion of filtrate + dil.HCl + BaCl2 (aq)	No visible reaction.	SO32-, CO32-, SO42- absent.
(ii) Portion of filtrate + dil.HNO3 + AgNO3(aq) + excess NH3(aq)	No visible reaction	Cl-, Br- , I- absent.
(iii) Portion of filtrate + NaOH(aq) in drops and then in excess	White gelatinous precipitate soluble in excess NaOH(aq)	Al3+, Zn2+ may be present
(v) Portion of residue + dil.HNO3	Effervescence of a colourless, odourless gas	CO2 gas from CO32- -
(vi) Portion of residue + dil.HCl. Resulting mixture heated and later cooled.	White precipitate dissolves on heating and re-appears on cooling.	Pb2+ present
(vii) Portion of residue + KI (aq)	Yellow precipitate	Pb2+ confirmed/present

(b)

Test

Observation

Inference

(i) Compound + NaHCO3(aq)

Effervescence/colourless gas
Turns limewater milky

Alkanoic acid present

(ii) Compound + H+/K2Cr2O7(aq) + heat

Orange solution turns green

Alkanol present