Note that, in the previous calculation, the activity coefficients have been ignored.

Lastly, find the fa for lime. The chemical reaction for lime after it has been slaked is

Begin by assuming a concentration 0.1 M. Let x be the concentrations of Ca2+ at equilibrium; the corresponding concentration of OH- will then be 2x at equilibrium. Thus,


Kp,ca ( oh)2 = 7.9( 10-6 ) = x ( 2 x )2 = 4 / x = 0.0125

Table 13.10 shows some other values of fa and fa.

x = -1.4(10-4) + V[1.4(10-4)f+ 4(1.0)[1.4(10-4)] = 00083

Example 13.6 A raw water containing 3 mg/L of manganese has a pH 4.0. To remove the manganese, the pH needs to be raised to 6.0. The current acidity is 30 mg/L as CaCO3 [Ca2+] = 0.7 mgmol/L. Assume the temperature is 25°C and of 0.0367. Calculate the amount of soda ash needed.


10 r B = 10 gmols/L; 10-pHcur = 10 gmols/L; ¥ = 1.0 m


MNa2CO3pH = 53.°{ + lJW36r }( 1 } = 0.175 kg/m3 Ans

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