3) A student mistakenly uses the cuvette containing the most dilute standard solution, rather than the cuvette containing deionized water, as the blank solution.
I don't understand why there would be lower concentration of Cr(VI) ion...
1 answer
When you use the spectrophotometer you insert the cuvette containing the blank and use the adjustment knob to make the needle read 100 %T. That is for no Cr solution in the beam. That adjustment takes care of any reflections from the liquid, from the walls of the cuvette, and it contains no Cr at all. When you insert the cuvette containing some Cr it will absorb some of the light and the %T will be less, say 80%T or 90%T. Let's suppose that the most dilute solution of Cr you have reads 90%T and you adjust the knob to make it read 100%T. So it should read 90 but you make it read 100. So now you put in a more concentrated solution it will read a higher %T also. You have moved the entire %T scale up by 10% (from 90 to 100). A higher %T means a lower absorbance and that means a lower concentration of Cr.