Engineering Chemistry S Datta

WATER TREATMENT 369
• Hard water does not easily lather with soap, instead it forms a greasy scum.
• Water is hard if it contains calcium or magnesium ions.
• Scum is a precipitate formed when soap comes in contact with hard water
containing above mentioned ions.
Ca 2+ (aq) + C 17
H 35
CO – 2
(aq) ⎯⎯→ Ca(C 17
H 35
CO 2
) 2
↓
insoluble precipitate
• Temporary hard water contains bicarbonates or hydrogen carbonates of calcium
or magnesium.
• Temporary hardness is removed by boiling.
∆
Ca(HCO 3
) 2
(aq) ⎯⎯→ CaCO 3
↓ + H 2
O(l) + CO 2
↑.
boiling
• The solid calcium carbonate is precipitated as scale, which affects heating element
of boilers and gradually blocks the pipes in heating system.
• Permanent hardness is not removed by boiling.
• The mineral gypsum (CaSO 4
) is slightly soluble in water and makes water
permanently hard.
• Ion exchange resin and other methods of softening of water by removing calcium
and magnesium ions is known as water treatment.
PROBLEMS
Problem 1. Calculate temporary hardness and total hardness of a sample of water
containing:
Mg(HCO 3
) 2
= 7.5 mg/l . Ca(HCO 3
) 2
= 16 mg/l
MgCl 2
= 9 mg/l . CaSO 4
= 13.6 mg/l
100 100
Sol. Temporary hardness = 7.5 × + 16 × mg/ l
146 162
L
NM
L
NM
100 100
Permanent hardness = 9 × + 13. 6 × mg/l.
95 136
Problem 2. How many mg. of FeSO 4
dissolved per litre gives 200 ppm of hardness?
Sol. (56 + 16 + 64)g of FeSO 4
≡ 100 g CaCO 3
For 100 ppm hardness FeSO 4
required per 10 6 parts of water is 136 parts.
O
QP
O
QP
∴ For 200 ppm hardness, FeSO 4
required =
136 × 200
100
= 272 mg/l.
ppm
Problem 3. A sample of water has a CO 3 2− concentration of 15.6 ppm. What is the molarity
of CO 3 2− in the sample of water?