Once the reaction has occurred as completely as possible, what mass (in g) of the excess reactant is left

How do i complete this electrolysis of water lab? i’m at home, so i don’t have the materials, and the lab didn’t properly work and was incomplete at school.

A50.0g sample of liquid water at 0.0 c ends up as ice at -20.0 c. how much energy is involved in this change?

An atom of which element reacts with an atom of hydrogen to form a bond with the greatest degree of polarity ?