Question 2B
F is a mixture of two salts.
Carry out the following exercises on F. Record your observation and identify any
gas(es) evolved. State the conclusion you draw from the results of each test.
(a) Put all of F in beaker and add about 10 cm3 distilled water.
Stir well and filter, if necessary.
(b) (i) To about 2cm3 portion of the solution, add few drops of Ba(NO3)2(aq)
followed by excess HNO3(aq).
(ii) To another 2 cm3 portion of the solution, add NaOH(aq) in drops then in excess.
(iii) Warm the resulting mixture from 2(b)(iii)
(iv) To another 2cm3 portion of the solution, add NH3(aq) in drops and then in excess.
(v) To another 2 cm3 portion of the solution, add few drops of potassium thiocyanate solution (KSCN (aq)).
Observation
The question tested quantitative analysis. Majority of the candidates performed above average.
In part (a), some candidates referred to filtrates as solutions or liquids. Some of them referred to residue as solid or precipitate.
In part (b), some candidates lost marks to references, due to the fact that their observations were not correctly reported.
Majority of the candidates were not familiar with KSCN solution. Consequently. They could not use it correctly.
Expected answers include:-
Question 2
|
TEST |
OBSERVATION |
INTFERENCE |
(a) |
F + distilled water + stir |
Dissolved to give yellow / yellowish brown / solution |
C contain soluble salts |
(b) |
F(aq)+ Ba (NO3)2(aq) +HNO3(aq) |
White (chalky) precipitate |
SO42-, SO32- , S2- |
(ii) |
F(aq) + NaOHaqdrops then in excess |
Reddish – brown gelatinuos precipitate |
Fe3+ present |
(iii) |
Mixture in b (ii) warmed |
Colourless pungent / irritating / choking gas evolved Gas turns red litmus blue/ Gas forms dense white fumes with Conc HCl |
Gas is NH3 |
(iv) |
F (aq) + NH3 (aq) in drops then in excess |
Reddish – brown (gelatinous) precipitate |
Fe3+ present |
(v) |
F (aq) + KSCN(aq) |
Deep red/blood red |
Fe3+ present /confirmed
|