Example 15-18, 19 Wastewater resulting from metal processing often contains sign

Question

Example 15-18, 19 Wastewater resulting from metal processing often contains significant amounts of toxic heavy metal ions that must be removed before the water can be returned to the environment. What mass of Cd2+ will dissolve in 1.00 x 10 L water? GIVEN : K (CdS) = 7.9 x 10-27 sp Ksp (Cd(OH)2) = 7.2 x 10-15 One method to remove Cd uses sodium hydroxide solution to precipitate insoluble metal hydroxides. Suppose that 1.00 x 10 L of wastewater containing 1.2 x 10-5 M Cd2+ is treated with n1 L of 6.0 M NaOH solution. What is the residual concentration of Cd2 after treatment and what mass of Cd precipitates?

Explanation / Answer

For Question 1:

Assume Cd(OH)2  decomposed to cd2+ and 2 (OH-). LEt equilibrium concentrations be x and 2x

Hence x*(2x)^2 = Ksp of Cd(OH)2

4x^3 = Ksp

x= 1.2 * 10^(-5)

Therefore 10^2 L water contains 10^2 * 1.2 * 10^(-5) = 1.2 *10^(-3) moles

Mass = Moles* MM = 1.2 *10^(-3) * 146 g/mol = 175.2 * 10^(-3) grams gets dissolved in water from cadmium hydroxide

Similarly calculate for Cadmium sulphide it is almost negligible.

Multiple Questions cannot be answered in single attempt.

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