Question

A groundwater is being considered for use as a domestic supply. The water has a temperature of 15°C and the following chemical analysis:

	Concentration g/m3
Ca2+	100
Mg2+	55
HCO3-	23
CO32-	50

a. Determine the hardness of the water, in mM.
b. Calculate the sample pH, using the carbonate species present in the water sample
c. Determine the total alkalinity of the water, in mM. (At 15°C, Kw = 4.46 x 10–15)
d. Would the water be an acceptable domestic supply in terms of its hardness and its pH?

Answer 1

Answer a): We know that ,

1mg/ml = 1 ppm

Concentration g/m3
Ca2+	100
Mg2+	55
HCO3-	23
CO32-	50

Hence , Total hardness will be = (100/100)+(55/24)+(25/62)+(50/62)= 4.5 mM

Answer 2): Using carbonate species:

[H+] = 2×10^-3 ×(50/62)=

pH= -log[ H+] 2.79

Answer 3): Total [H+]=[ (25/62)×10^-3]+[(2×10^-3)×(50/62)]

pH= -log[ H+] total~2.695

[H+] = 2×10^-3

[OH-][H+]= 4.46×10^-19(given value)

[OH-]= 2.2×10^-12 M(Total alkanility)

Answer 4):

No, water would not an acceptable domestic supply in terms of its hardness and its pH

Because of very low pH.

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