Calculate the Q sp or K sp , as indicated, and determine whether a precipitate w

Question

Calculate the Qsp or Ksp, as indicated, and determine whether a precipitate will form when each of the following mixtures is prepared. (You may assume temperature is held constant, volumes are additive, and that the initial solutions, prior to mixing, are unsaturated.)

(a) 25.24 mL 1.36???10?4M CaCl2 is mixed with 25.34 mL 3.23???10?3M NaF. (Ksp for CaF2 = 3.5???10?11)

Choose one: A precipitate will form or A precipitate will not form.

(b) 14.95 mL 3.65???10?3M Pb(NO3)2 is mixed with 34.89 mL 1.75???10?4M Na2SO4. (Ksp for PbSO4 = 2.5???10?8)

Choose one: A precipitate will form.A precipitate will not form.

(c) 50.27 mL 2.53???10?2M Pb(NO3)2 is mixed with 25.11 mL 4.68???10?5M NaBr. (molar solubility of PbBr2 = 1.2???10?2M)

Choose one: A precipitate will form.A precipitate will not form.

Explanation / Answer

Precipitation

(a) Qsp = [Ca2+][F-]^2

             = (1.36 x 10^-4 M x 25.24 ml/50.58 ml)(3.23 x 10^-3 M x 25.34 ml/50.58 ml)^2

             = 1.78 x 10^-10

Qsp > Ksp, so precipitation would occur

(b) Qsp = [Pb2+][SO4^2-]

             = (3.65 x 10^-3 M x 14.95 ml/49.84 ml)(1.75 x 10^-4 M x 34.89 ml/49.84 ml)

             = 1.34 x 10^-7

Qsp > Ksp, precipitation would occur

(c) Qsp = [Pb2+][Br-]^2

             = (2.53 x 10^-2 M x 50.27 ml/75.38 ml)(4.68 x 10^-5 M x 25.11 ml/75.38 ml)^2

             = 4.10 x 10^-12

Ksp = (1.2 x 10^-2)(2 x 1.2 x 10^-2)^2

       = 6.91 x 10^-6

Qsp < Ksp, no-precipitation would occur.

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