Observation

This question was popular among the candidates because hardness of water attracted them to the question

In part (a), majority of the candidates were able to explain hardness of water. However, they could not describe how to determine the proportion of hardness in a given sample of water is due to permanent hardness.

In part (b), majority of the candidates stated the reagents and conditions for the laboratory preparation of trioxonitrate (V) acid.

In part (c), majority of the candidates were able to name one amphoteric oxide.

.

The expected answers include:

(a) (i) When water does not easily form lather with soap / when water contains dissolved calcium and magnesium salts.

Water that does not easily lather with soap / Water that contains dissolved calcium and magnesium salts                                    

(ii) Prepare soap solution             
Measure volume of sample water from a burette or pipette titrate with shaking to permanent lather.
take titre value say x cm3

Measure same volume of boiled sample water and then titrate with shaking take titre value say y cm3

                                                         
(iii) Waste of soap
-  produces scum during washing with soap
-  blockage of pipes
-  scales in boilers / furring of kettles
-  leads to wastage of fuel
-  not suitable for tanning / laundry / textile/ paper manufacturing industries
                                                                       
(b) (i) NaNO3 or KNO3           
conc H2SO4                 

(ii) concentrated H2SO4
heat (over a sand bath)
all glass apparatus

(iii)   2HNO3 + S → H2SO4 + 2NO            
OR
6HNO3 + S → H2SO4 + 6NO2 + 2H2O

 II. no reaction / reaction prevented / metal passive oxide formed which
coats metal                                                                                                      
(c)  Zinc oxide
Lead (II) oxide

Aluminium oxide
Tin(IV)oxide
Aluminium hydroxide
Tin (II) oxide
Lead (IV) oxide
Beryllium oxide