A solution is 5 mM in each of the following ions: number ion Ksp of M(OH)2 1 Mg2

Question

A solution is 5 mM in each of the following ions: number ion Ksp of M(OH)2 1 Mg2+ 1.8e-11 2 Cd2+ 2.5e-14 3 Co2+ 1.6e-15 4 Zn2+ 4.4e-17 5 Cu2+ 2.2e-20 Indicate which of the metal ions would precipitate (or start to precipitate) at each of the following pH values. Indicate your answer with the number of the ion. Use 0 to indicate no precipitate. If more than one precipitate is expected, list the numbers in increasing order and separate them with commas. For example, 3,4,5 is ok but 5,4,3 is not. pH = 7.00: pH = 9.00: What is the pH to the nearest 0.1 pH unit at which Cd(OH)2 begins to precipitate? pH =

Explanation / Answer

for solubility at pH5, for 5mM solutions of all, x = 0.005M for Mg+2 pOH = 14 - 5 = 9 [OH-]= 10^-9 = 1 x 10^-9 M QMg+2 = 0.005M x (1x10^-9)^2 = 5 x 10^-21, QKsp, ppt ppt forms for all ions Mg(OH)2 will ppt out at pH 10 but Q is very close to the Ksp value which when Q=Ksp, the solution is saturated with no ppt for pH of Co(OH)2 ppt formation Q = Ksp at saturation 1.6x10^-15 = (0.005) x [OH-]^2 [OH-] = 5.66 x 10^-7M pOH = log[OH-] = 6.24 so pH = 7.75

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