Chemistry Paper 3A WASSCE (SC), 2016

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
(Fe3+ may be present)

(b)
(i)

F(aq)+ Ba (NO3)2(aq)

           +HNO3(aq)

White (chalky) precipitate
Precipitate insoluble

SO42-, SO32- , S2-
CO32-
SO42- present

(ii)

F(aq) + NaOHaqdrops

then in excess

Reddish – brown gelatinuos precipitate
Precipitate insoluble

 

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
NH4+  present

(iv)

F (aq) + NH3 (aq) in drops

then in excess

Reddish – brown (gelatinous) precipitate
Precipitate insoluble

 

Fe3+ present

(v)

   F (aq) + KSCN(aq)

Deep red/blood red
solution formed
(accept red solution )

Fe3+  present /confirmed