Question

Suppose that a 2.50x10^-6M solution of red dye No. 2 had an absorbance of 0.585 at its wavelength of maximum absorbance. A second solution of red dye No. 2 is too concentrated to read its absorbance so 5.00 mL of the concentrated dye solution was diluted with water to a volume of 150 mL. The diluted solution has an absorbance of 0.289, measured under identical conditions. What is the molarity of dye in the concentrated solution?

Answer 1

For the first one,
Absorbance = kc
A = 0.585, solve for k, c =- 2.50 x 10^-6M

Use k in the second run of the concd stuff.
A = kc
You will get a concn that must be adjusted because of the dilution. How much was it diluted. From 5 to 150 mL; therefore, the concn will whatever you calculate x the factor 150/5.
Or you can look at it another way. You can say that the absorbance would have been higher than 0.289 by 0.289 x (150/5), then work A = kc the normal way.