beers law problem

Question

During an experiment, a chemistry student collects absorbance vs concentration data for different concentrations of a salt solution at 490 nm. the experiment is run using a 1.00 cm diameter cuvette and the same spectrophotometer. what procedural errors best accounts for a absorbance reading that is too high?

the answer is: when the salt solution was prepared, an extra 0.4 g of salt was added ( and it all dissolved).

I picked: when the salt solution was prepared, not all of the solid dissolved.

I did not pick the correct answer because I thought when it all dissolved, it would just be in ion form and that would not affect the light going through the container. I picked the wrong answer because I thought if the solid is not dissolved, the solid would cause the light to not shine through as much and as a result a higher absorbance. Why is my answer wrong and why is the correct answer correct?

Answer 1

The absorbance is the amount of light that is absorbed by the SOLUTION. Any solid material would essentially just sit at the bottom of the cuvette and would not affect the absorbance or the transmittance. Now it is true that if the undissolved material were in suspension, then it might affect transmittance/absorbance.

The correct answer is that the solution was prepared incorrectly by adding too much salt. This will raise the concentration of salt in solution and according to Beers law, this will increase the absorbance.

Hope the helps explain why your answer was incorrect.