Question 4
(a) (i) What is meant by hardness of water?(ii)Describe briefly how you would determine what proportion of hardness in a given sample of water is due to permanent hardness.
(iii) Give two reasons why hardness of water is an undesirable property. [15 marks]
(b) State the:
(i) reagents;
(ii) conditions for the laboratory preparation of trioxonitrate (V) acid.
(iii) How does concentrated trioxonitrate (V) acid reacts with:
(I) sulphur;
(II) aluminium.[9 marks]
(c) Name one amphoteric oxide. [1 mark]
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) I 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
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) I 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