Answer:
(a) Avoid parallax error
(b) To ensure that almost all the SCN⁻ has reacted.
Explanation:
(a) Avoiding bias
A Mohr pipet is a graduated pipet. To avoid bias, you should always hold the pipet so the meniscus is at eye level.
If your eye is above or below the meniscus, you will get an incorrect reading of the volume.
(b) Excess Fe³⁺
I am guessing that you are adding different volumes of SCN⁻ to Fe³⁺ to generate a Beer's Law plot of the concentration of FeSCN²⁺.
The problem is that the reaction is an equilibrium It does not go to completion.
Fe³⁺ + SCN⁻ ⇌ FeSCN²⁺
You use a large excess of Fe³⁺ to drive the position of equilibrium to the right and make sure that almost all the added SCN⁻ is converted to FeSCN²⁺.
