Question

1. What is the %T for a sample if its absorbance is 1.27?

2. The accuracy of a spectrophotometer is evaluated by preparing a solution of 60.06 ppm K2Cr2O7 in 0.0050 M H2SO4, and measuring its absorbance at a wavelength of 350 nm in a cell with a pathlength of 1.00 cm. The expected absorbance is 0.640. What is the expected molar absorptivity of K2Cr2O7 at this wavelength?

Answer 1

1. The relationship between transmittance (T) and absorbance (A) is given by:

A = -log(%T/100)

We can therefore calculate the %T by re-arranging and solving:

5.37%.

---

2. The relationship between absorbance and concentration is given by the Lambert-Beer law, which states that:

Where epsilon is the molar absorptivity and l is the optical path length. We can calculate epsilon by re-arranging and solving, but we first need to know the concentration of dichromate in molar units, since we are required to calculate the molar absorptivity:

Now we convert to molarity using the molar mass (249.18 gr/mol):

And now we can calculate the molar absorptivity: