Question 2B
F is an organic solid. Carry out the following exercises on F. Record your observations and identify any gas(es) evolved. State the conclusions you draw from the result of each test.
Divide F into two equal portions.
Put the first portion of F into a dry boiling tube and heat strongly.
Put the second portion of F into a test tube and add about 10 cm3 of dilute HNO3. (Filter if necessary).
To about 2 cm3 portion of the resulting solution:
add dilute NaOH in drops and then in excess;
add dilute NH3 in drops and then in excess;
add HCl(aq).
From your results, identify the ions in F. [21 marks]
Observation
This question was well attempted by majority of the candidates and their performance was fair.
In part (a), some of the candidates did not report their observations and inferences in the acceptable format. Also, majority of them wrote wrong charges and inferences.
In part (b), majority of the candidates did not follow the instructions. Hence, they recorded wrong observations.
In part (c), majority of the candidates could not identify the ions in F.
The expected answers include:
|
TEST |
OBSERVATION |
INFERENCE |
(a)(i) |
F + Heated strongly
|
Colourless gas evolved turns lime water milky |
CO2 from |
(ii) |
F + dil HNO3 |
Effervescence colourless odourless gas evolved . Gas turns lime water milky. |
CO2 from |
(b)(i) |
Resulting solution from a(ii) + NaOH(aq) in drops |
White (chalky) precipitate Precipitate soluble |
Pb2+ or Ca2+ Pb2+ |
(ii) |
Resulting solution + then in excess |
White (chalky) precipitate
Precipitate insoluble/No visible change |
Pb2+ present |
(iii) |
Resulting solution |
White precipitate |
Pb2+ confirmed |
(c) The ions present in F are Pb2+ and CO32-