Question

b)An acidic solution containing La3 + concentration 0.010 mol L-1 is treated with NaOH until the...

b)An acidic solution containing La3 + concentration 0.010 mol L-1 is treated with NaOH until the precipitation of La (OH) 3. The pH at which precipitation of the hydroxide should start is: Data: pKps (La (OH) 3) = 20.7; Kw = 1,0x10-14. (Explain your answer)

(A) 4.7. (B) 6,2. (C) 6.7. (D) 7.8.

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Answer #1

answer : (D) 7.8

Ksp = 10^-pKsp = 10^-20.7 = 1.995 x 10^-21

La(OH)3    ------------------> La+3 + 3 OH-

                                            0.01       

Ksp = [La+3][OH-]^3

1.995 x 10^-21 = 0.01 [OH-]^3

[OH-] = 5.84 x 10^-7 M

pOH = -log [OH-] = -log (5.84 x 10^-7)

         = 6.2

pH + pOH = 14

pH = 14 - 6.2

pH = 7.8

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