Question

1.a What would be the absorbance of 2.63x10^-5M tris at 508nm in a 2.0cm cell if the molar absorptivity of the complex is 1.10x10^4?

Suppose that the iron complex in problem 1a was 1.0% dissociated and the free 1,10 phenantholine had a molar absorptivity of 1.25x10^3 at 508nm

1. Calculate the measured absorbance of the solution. 2. Calculate the percent error in absorbance. 3. What molar concentration of iron complex would be calculated from the measured absorbance if it was assumed that no dissociation occured? 4. What is the percent error in molar concentration of iron complex

Answer 1

1a. Formula: A = cl

i.e. A = 1.10x10⁴ M^-1cm^-1 * 2.63x10^-5 M * 2 cm = 0.5786

If the complex is 1% dissociated, then the concentration of the complex = (1/100)*2.63x10^-5 = 2.63x10^-7 M

The molar absorptivities are additive in nature, therefore, = 1.10x10⁴ + 1.25x10³ = 11.25*10³ M^-1cm^-1

Now, 1. A = 11.25*10³ M^-1cm^-1 * 2.63x10^-7 M * 2 cm = 5.9175*10^-3

2. The % error in the absorbance = (5.9175*10^-3/0.5786) * 100 = 1.023%

3. The molar concentration of iron complex that would be calculated from the measured absorbance if it was assumed that no dissociation occured ≠ 0

4. The %error in the molar concentration of iron complex depends on the exact non-zero value of its concentration.