Question

Which of the combinations below will not produce any precipitate? The concentrations shown are after mixing, but before reaction. 6.58e-2 M Mg(NO3)2 + 3.53e-4 M NaF; Ksp (MgF2) = 6.46e-9. 3.63e-9 M Pb(NO3)2 + 8.97e-10 M Na3PO4; Ksp (Pb3(PO4)2) = 3.16e-44. 9.10e-18 M AgNO3 + 1.33e-21 M Na2S; Ksp (Ag2S) = 6.31e-51. 1.06e-7 M Zn(NO3)2 + 3.28e-8 M Na3PO4; Ksp (Zn3(PO4)2) = 1.00e-36.

Answer 1

for precipitate

Q > KSp

6.58e-2 M Mg(NO3)2 + 3.53e-4 M NaF; Ksp (MgF2) = 6.46e-9.

Q = (Mg+2)(F-)^2 = (6.58*10^-2)(2*3.53*10^-4)^2 = 3.27*10^-8

since Q > Ksp, there is precipitate

3.63e-9 M Pb(NO3)2 + 8.97e-10 M Na3PO4; Ksp (Pb3(PO4)2) = 3.16e-44.

Q = [Pb+2]^3[PO4-3]^2

Q = (3.63*10^-9)^3 (8.97*10^-10)^2 = 3.848 *10^-44

it is pretty near, Q > Ksp slightly, expect almost no precipitate

9.10e-18 M AgNO3 + 1.33e-21 M Na2S; Ksp (Ag2S) = 6.31e-51.

Q = Ag^2 * S

Q = (9.1*10^-18)^2(1.33*10^-21) = 1.101 *10^-55

Q << KSp, no precipitation

1.06e-7 M Zn(NO3)2 + 3.28e-8 M Na3PO4; Ksp (Zn3(PO4)2) = 1.00e-36.

Ksp = (Zn+2)^3(PO4-3)^2

Ksp = (1.06*10^-7)^3 (3.28*10^-8)^2

Ksp = 1.2813*10^-36

Q << Ksp, slightly, so expect no precipitation