When a student doesn't polish a sample of Mg ribbon before massing it, what happens when calculating the molar volume? Is it that the Mg ribbon doesn't completely react therefore less volumes of gas are produced?

Answer 1

"Old" magnesium has a thin coating of MgO on the surface. The purpose of polishing is to remove the oxide coating. If you don't remove the oxide coating then the mass of "real" Mg is too high (due to the oxide coating being weighed along with the Mg) and less gas is evolved than should be.

Answer 2

Thanks :) so just to be clear, does it matter what the Mg is reacting with, or is it always going to have the coating of the MgO on it?

Answer 3

In air the coating will be MgO. In some other atmosphere it may be something else.