Question

Calculate the Qsp or Ksp, as indicated, and determine whether a precipitate will form when each of the following mixtures is prepared.

(a) 25.12 mL 1.57 ✕ 10−4 M CaCl2 is mixed with 25.13 mL 3.26 ✕ 10−3 M NaF. (Ksp for CaF2 = 3.5 ✕ 10−11)

(b) 14.77 mL 3.68 ✕ 10⁻³M Pb(NO₃)₂ is mixed with 35.01 mL 1.63 ✕ 10⁻⁴M Na₂SO₄. (K_sp for PbSO₄ = 2.5 ✕ 10⁻⁸)
(c) 50.33 mL 2.62 ✕ 10⁻²M Pb(NO₃)₂ is mixed with 25.20 mL 4.83 ✕ 10⁻⁵M NaBr. (molar solubility of PbBr₂ = 1.2 ✕ 10⁻²M)

Answer 1

a)

concentration of Ca2+ = 25.12 x 1.57 x 10^-4 / 25.12 + 25.13

                                    = 7.85 x 10^-5 M

concentration of F- = 25.13 x 3.26 x 10^-3 / 25.12 + 25.13

                                = 1.63 x 10^-3 M

Qsp = [Ca2+][F-]^2

      = (7.85 x 10^-5) (1.63 x 10^-3)^2

Qsp = 2.1 x 10^-10

Ksp = 3.5 x 10^-11

here Qsp > Ksp. so yes , precipitate will form

b)

concentration of Pb2+ = 14.77 x 3.68 x 10^-3 / 14.77 + 35.01

                                 = 1.09 x 10^-3 M

concentration of SO42- = 35.01 x 1.63 x 10^4 / 14.77 + 35.01

                                        = 1.146 x 10^-4 M

Qsp = [Pb2+] [SO42-]

      = (1.09 x 10^-3) (1.146 x 10^-4)

Qsp = 1.2 x 10^-7

here Qsp > Ksp. so yes , precipitate will form

(c)

concentration of Pb2+ = 0.0175

concentration of Br- = 1.61 x 10^-5

Q = [Pb2+] [Br-]^2

Qsp = 4.5 x 10^-12

here Qsp < Ksp . so precipitate will not form