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

Question

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

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If 18.0 mL of 4.86×10-4 M Pb(NO3)2 are added to 15.0 mL of 4.20×10-5 M NaCl, will solid PbCl2(Ksp = 1.7×10-5) precipitate?   _____. yes. no.

If a precipitate will not form, what chloride ion concentration will cause a precipitate of lead chlorideto 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

i) Solubility equillibrium of PbCl2 is

PbCl2(s) <-------> Pb2+(aq) + 2Cl-(aq)

Ksp = [Pb2+][Cl-]2 = 1.7×10-5

Total volume = 18ml + 15ml = 33ml

Dilution factor for Pb2+ solution = 33ml/18ml = 1.8333

[Pb2+]= 4.86×10-4M/1.8333 = 2.65×10-4M

Dilution factor of Cl- solution = 33ml /15ml = 2.2

[Cl-]= 4.20×10-5M/2.2 = 1.91×10-5

Qsp = [Pb2+][Cl-]2

Qsp = 2.65×10-4M ×( 1.91×10-5M)2

Qsp = 9.67×10-14

Qsp < Ksp

Therefore,

No, precipitation will not form

ii)

Ksp = [Pb2+][Cl-]2

[Pb2+][Cl-]2 = 1.7×10-5M3

substituting concentration of Pb2+

2.65×10-4× [Cl-]2 = 1.7×10-5M3

   [Cl-]2 = 1.7×10-5M3/2.65×10-4M

[Cl-]2 = 6.42×10-2M2

     [Cl-] = 0.2534M

Therefore,

the minimum concentration of Cl- will cause a precipitate = 0.2534M

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