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)
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

Answer 1

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