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...

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.

The mistake of using the cuvette containing the most dilute standard solution instead of deionized water as the blank solution can lead to incorrect results. This is because the blank solution is used to calibrate the spectrophotometer and to account for any background absorbance. It should ideally have no analyte of interest, in this case, the Cr(VI) ion.

When a blank solution with the most dilute standard solution is used, it means that there is some concentration of the Cr(VI) ion present in the blank solution. As a result, the spectrophotometer will detect some absorbance in the blank solution, which should have ideally been zero.

This mistake can lead to an overestimation of the concentration of Cr(VI) ion in the subsequent samples. The absorbance measured in the samples will have contributions from both the Cr(VI) ion in the samples and the Cr(VI) ion in the mistakenly used blank solution.

To rectify this mistake, the student should redo their analysis by using the correct blank solution, which is deionized water. The deionized water should not contain any analyte that might interfere with the measurement. By using the correct blank solution, the spectrophotometer can be properly calibrated to measure the absorbance of only the Cr(VI) ion in the samples, allowing for accurate determination of its concentration.