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Theory
If Br- ions are added to any solution containing Ag+ ions, AgBr cannot precipitate until the product of the ion concentrations, (Ag+) (Br-), exceeds the solubility product constant, Ksp, for AgBr.A solution that contains NH molecules in high concentration can only contain Ag+ ions in very low concentration because NH molecules and Ag+ ions unite to form a complex with a large formation constant.If we represent the formation of the complex by the equation,
Ag+ + nNH = Ag(NH)n+(1)
Then the expression for the formation constant (equilibrium constant with the concentration of the complex in the numerator) is
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Kf =[Ag(NH)n+]()
[Ag+] [NH]n
Since the formation constant of the aminesilver complex has a large numerical value, a high concentration of Br- ions must be established in order to produce a AgBr precipitate from a solution of "ammoniacal silver nitrate."At the point at which the precipitate first forms we can calculate the concentration of the umcomplexed Ag+ ions if we nkw the concentration of the Br- ions.The numerical value of Ksp for AgBr is required for this calculation.However, to determine n in the formula for the complex it is only necessary to recognize that [Ag+] is inversely proportional to [Br-].
Apparatus
4-100 mL graduated cylinders
pipette
AgNO
KBr
4 erlenmeyer flasks
buret
To each of the four 100-mL graduate cylinders add, respectively, 10.0 mL, 15.0 mL, 0.0 mL, and 5.0 mL of .0 M ammonia solution.Into each cylinder we pipette 0.0 mL of 0.010 M AgNO solution which was prepared by weighing our AgNOcrystals and dissolving them with distilled water in a 50 L volumetric flask.We then diluted each of the resulting ammoniacal silver nitrate solutions to the 00-ml mark with distilled water.We then transfered the contents of each cylinder to an Erlenmeyer flask and titrate with 0.010 M KBr solution which was prepared by weighing out KBr crystals and dissolving them with distilled water in a 100 L volumetric flask until a pemanent precipitate is just formed.These preliminary titrations were calculated to the nearest mL.The approximate required volume of KBr solution T'( the approximate "titre").
Results and Calculations
Table 1Calculations for Formula of a Complex
ML of AgNOML of .0M NH VML of KBrT'ML of KBrTLog VLog T
010.0.01.0000.4771
015.5.51.1760.5441
005.55.51.010.7404
056.56.51.80.81
14.8M x X = .0M x 50mL
X = .78 mL of NH4OH
0.010M AgNO x 0.50 L x 170g = 0.45g AgNO
0.010M KBr x 0.100 L x 11g = 0.11g KBr
Discussion
Since a large excess of ammonia is used, the equilibrium expressed by the equation (1) is driven almost driven completely to the right.Therefore, the concentration of the silver ammine cationic complex, [Ag(NH)n+], is virtually constant in all four experiments.Furthermore, the concentration of the NH molecules, [NH], is virtually unchanged by the titration.We determined the n by plotting log T against log V.
References
1.Noggle, Joseph N."Physical Chemistry," d ed., Harper Collins College
Publishers, New York, 16.
.Skoog, Douglas A."Analytical Chemistry" 7th ed., Saunder College Publishing,
New York, 16.
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