Use the reaction quotient to predict whether a precipitate will form. If 15.0 mL

Question

Use the reaction quotient to predict whether a precipitate will form.

If 15.0 mL of 2.40×10-4 M Pb(NO3)2 are added to 22.0 mL of 9.20×10-5 M NH4Cl, will solid PbCl2(Ksp = 1.7×10-5) precipitate?   _____(yes or no)

If a precipitate will not form, what chloride ion concentration will cause a precipitate of lead chloride to form? If a precipitate will form, what is the minimum [Cl-] that could have been present without initiating precipitation? Assume the total volume used in the above example.

______ M

Explanation / Answer

1) No

For precipitation

Qsp > Ksp

[Pb(NO3)2] = 2.40× 10^-4M

[Pb2+ ] = 2× 2.40 × 10^-4M = 4.8 × 10^-4M

After mixing the two solutions ,volume = 37ml

Dilution = 37/15 = 2.47

Final [Pb2+] = 1.94× 10^-4M

[NH4Cl] = 9.20 ×10^-5M

[Cl-] = 9.20× 10^-5M

Final valume 37ml

Dilution = 37/22= 1.68time

[ Cl^-] = 5.48 × 10^-5

Qsp = [Pb2+][Cl-]^2

=1.94×10^-4 × (5.48 ×10^-5)^2

= 5.8 × 10^-14

Ksp =1.7 × 10^-5

Qsp < Ksp

Precipitation will not form

2) Ksp = 1.7 × 10^-5

Ksp = [ Pb2+] [ Cl-]^2

1.7 × 10^-5 =1.94×10^-4 × [Cl-]^2

[Cl-]^2= 1.7×10^-5/1.94×10^-4

[Cl-] = 0.297M

So, precipitation will form > 0.297 M of Cl-

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