Molar Absorptivity and Beer's Law

4. A weak organic acid in its dissociated form (A-) absorbs at 375 nm with a molar absorptivity, ε, of 4.48×103 L/(mol?cm). The non-dissociated form of the acid (HA) does not absorb at 375 nm. At 375 nm, a 3.0×10-3 M solution of the acid in water has an absorbance of 0.462 in a 1.00 cm cell.
a. What is the concentration of A- in this solution?
b. What is Ka for this acid?

5. The molar absorptivity for the complex formed between bismuth (III) and thiourea is 9.32 x 103 L mol-1 cm-1 at 470 nm. Calculate the range pf permissible concentrations for this complex if the percent transmittance is to be greater than 15.0 % and less than 85.0 % in a 2.00-cm cell.

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...- in this solution?
b. What is Ka for this acid?

To solve for concentration, you need to use Beer's law:

Here, e is molar absorptivity, l is the length of the cell, and c is the concentration (in M).

Now, to solve for Ka, you can set up an equilibrium:
HA  H+ + A-

The concentration of H+ and A- will be roughly the same. You can plug ...