Chem Problem(40marks for best ans)

1.
Io = intensity of a monochromic light after passing through the reference (a 「blank」sample solution). This is set to 100% transmittance.
I = intensity of the monochromic light passing through the coloured solution.
In ideal case, the two test tubes (to hold the reference and to hold the coloured sample solution) should be identical. This makes sure that I/Io is only due to the absorption of light by the coloured solution.
If the two test tubes are significantly different (e.g. in thickness of glass, curvature, refractive index, and even the presence of stain and fingerprint, etc.), I/Io will NOT be only due to the absorption of light by the coloured solution, but also due to the difference between the test tubes. This will cause a significant error.

To avoid the significant difference between test tubes, the test tubes used must be matched.
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2.
The principle of choosing a filter is 「to select a particular wavelength of light for the coloured solution to absorb」.
Ideally, the filter should let through ONLY light of the particular wavelength absorbed by the coloured solution. So, for a red solution which absorbs cyan light, we would use a cyan filter - cyan is the complementary colour to red. The following shows complementary colour pairs.
red - cyan, green - magenta, blue - yellow
For example, in the experiment of the reaction between iodine and propanone in aqueous solution, since iodine is reddish, we would use a filter in the blue-green range.

1. If the tubes are not matched, the color may not be matched and it will lead to deviation.

2. If the color of the reaction is very intense, we have to use filter to lower teh intensity. Otherwise, the detector may be saturated and the result may not follow the Beers Law. An error may be induced. Also, if the reaction has some background interference, which may mask teh peak, we have to use the filter to make sure that the change of color is not masked.