1. The following is the equation for the formation of the ammonia complex of Cu^

Question

1. The following is the equation for the formation of the ammonia complex of Cu^+2

Cu^2+(aq) + 4NH2(aq)? Cu(NH3)4^2+(aq)

if at equilibrium the concentration of Cu^2+ is 1.0 x 10^-3 M, the concentration of NH3 is 1.0x10^-2 M and the concentration of Cu(NH3)4^2+ is 1.7x10^2 M What is the value of K ?

2. Cu2+(aq) have a molar absorptivity of 12.7 M^-1 cm^-1 at 645nm. If a solution of Cu^2+ of unknown concentration has an absorbance of 0.418 measured in a 1.00cm path length cell at 645 nm , what is the concentration of Cu^2+ ?

3. Explain why in order to get a Beer's Law plot of absorbance vs [FeNCS^2+] the [Fe^3+] had to be in vast excess of the concentration of the SCN ?

Explanation / Answer

1.Cu^2+(aq) + 4NH3(aq)? Cu(NH3)4^2+(aq)

K = [Products] / [Reactants]

= [Cu(NH3)4^2+] / [Cu^2+(aq)] * [NH3(aq)]^4

= 1.7 * 10^13

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3.A problem arises in determining the concentrations of the complex ion to use in the Beer's Law plot. Since the reaction forming the ion is an equilibrium process, it does not proceed to completion; solutions of the complex ion will generally also contain measurable concentrations of the two reactant ions, and the concentration of the complex ion will not be known. However, we can make use of Le Chatelier's Principle to overcome this obstacle.

If a limited quantity of iron(III) ion is combined with a large excess of thiocyanate ion, the equilibrium will be shifted far to the product side, essentially to completion. The number of moles of the complex ion formed will then be essentially equal to the number of moles of iron(III) ion that were used initially. A series of five different dilutions of the iron(III) ion will be used to prepare solutions of the complex ion in accord with this principle and a Beer's Law plot will be made in order to determine the extinction coefficient of the complex ion.

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2.Beer-Lambert Law
Relationship Between Molar Concentration and Absorbance

Solution colour results from the absorbance of some light wavelengths by solutes dissolved in solution, while allowing other wavelengths to pass through (transmittance). The combination of the remaining wavelengths that pass through results in the colour of the solution.

A colorimeter can be used to determine the amount of light at a particular wavelength that is absorbed/transmitted by a solution. Depending on the concentration of the solute in the solution, more or less absorbance/transmittance will result.

A = X * l * c

whereA is the absorbance (no units)
l is the length of the path (cm)
c is the molar concentration (mol/L)
X is the molar absorptivity at a specific wavelength

therefore

COncentration

C = A / X * l

= 0.418 / [ 12.7 M^-1 cm^-1 ] * [1.00cm]

= 0.0329 moles / liter

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