Question

BLindlar catalyst/H2→R−C≡C−RNa/NH3→A. A and B are geometrical isomers (R−CH=CH−R)

Answer 1

Explanation:

Measuring changes at lambda max ensures that you're getting the most accurate change in absorbtion, and thereby get a more accurate value for the concentration. Just to add on. The other thing is that the peak maximum is also one of the easiest points on the graph to report in literature.