Question

A 75 L solution of 0.63 M Fe(NO3)3 is mixed with 89 mL of 0.33 M Ni(NO3)2. Ksp Fe(OH)3=1.63x10^-39 and Ksp Ni(OH)2 = 6x10^-16

(a) If solid KOH is added to the solution, what will precipitate first? Give the chemical formula.

(b) If solid KOH is added to the solution, at what concentration of [OH-] will one begin to see the first sign of precipitate?

(c) At what concentration of [OH-] will one begin to see the first sign of precipitation of the second complex?

(d) What is the chemical formula for this second substance precipitating?

Answer 1

My answer is,

a. If solid KOH is added to a mixture of 75 L of 0.63M of Fe(NO₃)₃ and 85 mL of 0.33M of Ni(NO₃)₂, then, Fe(OH)₃ will precipitate out first as it is more saturated.

b. The concentration of [OH-] at which the first precipitate occurs

     1.63 x 10^-39 = X x 0.63 M

      X = 2.587 x 10^-39 M

c. The concentration of [OH-] at which the second precipitation occurs

    6 x 10^-16 = X x 0.33M

      X = 18.18 x 10^-16 M

d. Chemical formula for second substance precipitating is Ni(OH)₂.