A solution contains 7.61×10 -3 M sodium chromate and 1.37×10 -2 M sodium chlorid

Question

A solution contains 7.61×10-3 M sodium chromate and 1.37×10-2 M sodium chloride.
Solid silver nitrate is added slowly to this mixture.

What is the concentration of chloride ion when chromate ion begins to precipitate?
[chloride] = --------------M

Table of Solubility Product Constants (Ksp at 25 oC)

Table of Solubility Product Constants (Ksp at 25 oC)

Type Formula Ksp Bromides PbBr2 6.3 × 10-6 AgBr 3.3 × 10-13 Carbonates BaCO3 8.1 × 10-9 CaCO3 3.8 × 10-9 CoCO3 8.0 × 10-13 CuCO3 2.5 × 10-10 FeCO3 3.5 × 10-11 PbCO3 1.5 × 10-13 MgCO3 4.0 × 10-5 MnCO3 1.8 × 10-11 NiCO3 6.6 × 10-9 Ag2CO3 8.1 × 10-12 ZnCO3 1.5 × 10-11 Chlorides PbCl2 1.7 × 10-5 AgCl 1.8 × 10-10 Chromates BaCrO4 2.0 × 10-10 CaCrO4 7.1 × 10-4 PbCrO4 1.8 × 10-14 Ag2CrO4 9.0 × 10-12 Cyanides Ni(CN)2 3.0 × 10-23 AgCN 1.2 × 10-16 Zn(CN)2 8.0 × 10-12 Fluorides BaF2 1.7 × 10-6 CaF2 3.9 × 10-11 PbF2 3.7 × 10-8 MgF2 6.4 × 10-9 Hydroxides AgOH 2.0 × 10-8 Al(OH)3 1.9 × 10-33 Ca(OH)2 7.9 × 10-6 Cr(OH)3 6.7 × 10-31 Co(OH)2 2.5 × 10-16 Cu(OH)2 1.6 × 10-19 Fe(OH)2 7.9 × 10-15 Fe(OH)3 6.3 × 10-38 Pb(OH)2 2.8 × 10-16 Mg(OH)2 1.5 × 10-11 Mn(OH)2 4.6 × 10-14 Ni(OH)2 2.8 × 10-16 Zn(OH)2 4.5 × 10-17 Iodides PbI2 8.7 × 10-9 AgI 1.5 × 10-16 Oxalates BaC2O4 1.1 × 10-7 CaC2O4 2.3 × 10-9 MgC2O4 8.6 × 10-5 Phosphates AlPO4 1.3 × 10-20 Ba3(PO4)2 1.3 × 10-29 Ca3(PO4)2 1.0 × 10-25 CrPO4 2.4 × 10-23 Pb3(PO4)2 3.0 × 10-44 Ag3PO4 1.3 × 10-20 Zn3(PO4)2 9.1 × 10-33 Sulfates BaSO4 1.1 × 10-10 CaSO4 2.4 × 10-5 PbSO4 1.8 × 10-8 Ag2SO4 1.7 × 10-5 Sulfides CaS 8 × 10-6 CoS 5.9 × 10-21 CuS 7.9 × 10-37 FeS 4.9 × 10-18 Fe2S3 1.4 × 10-88 PbS 3.2 × 10-28 MnS 5.1 × 10-15 NiS 3.0 × 10-21 Ag2S 1.0 × 10-49 ZnS 2.0 × 10-25 Sulfites BaSO3 8.0 × 10-7 CaSO3 1.3 × 10-8 Ag2SO3 1.5 × 10-14

Explanation / Answer

A solution contains 7.61×10-3 M sodium chromate and 1.37×10-2 M sodium chloride.
Solid silver nitrate is added slowly to this mixture.

What is the concentration of chloride ion when chromate ion begins to precipitate?

Na2CrO4 = 7.61*10^-3 M

NaCl = 1.37*10^-2 M

AgNO3 --> Ag+

Ag2CrO4 vs. AgCl

from Ksp

Ksp = [Ag+]^2 * [CrO4-2]

9*10^-12 = [Ag+]^2 * (7.61*10^-3 )

[Ag+] = ((9*10^-12) / (7.61*10^-3 ))^0.5 = 0.0000343 = 3.43*10^-5 M

for

Ksp = [Ag+][Cl-]

1.8*10^-10 = [Ag+](1.37*10^-2)

[Ag+] = (1.8*10^-10)/((1.37*10^-2)) = 1.3138*10^-8 M

Clearly,

AgCl will precipitate first

after AgCl precipitates...

[Ag+] = ((9*10^-12) / (7.61*10^-3 ))^0.5 = 0.0000343 = 3.43*10^-5 M will be the [Ag+] value for CrO4-2 precipitation

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