What observations support the assumption that all of the iodine reacted with the zinc?

The answer is That percentage of composition of Zinc as determined said to be 19%, the percentage of iodine 81% and it shows that it is very close to accepted values by making calculations.

Explanations:

When iodine is dissolved in a strongly polar substance such as menthol. As the reaction is progressed, the color of the solution going from its initial color. Furthermore, after the reaction, small grey pallets supports the percentage between iodine reacted with the zinc. All the iodine reacted whereas portion reacted while the rest remained untouched.