ocr trends and patterns question colorimetry

For the colorimetry, when you find out the ratio of moles between a ligand and a metal ion. What is the reason why you have to top the solution with water to make to the solution an equal volume.

For example: Volume of Fe3+ 5 5 5 5 5 5
------------ Volume of SCN- 2 4 6 8 10 12
----------- Volume of water 13 11 9 7 5 3

You probably cant see the table good, but all the solutions add up to 20cm3.
I would like a detailed answer plz, i just have a feeling there gonna ask it in the exam :smile:

Thx

Reply 1

silent ninja

You want to to keep the test fair. If you have unequal volumes of solution you are altering the concentrations, so you're readings on the colorimeter are well..useless. So you make up your solutions and top up with distilled water to keep the total volume the same.