If a mole of hydrogen reacts with a mole of oxygen to produce water, what is the excess reactant?

Oxygen is the excess reactant.

Explanation: A simple balanced chemical equation for the production of water, will show:

2H2 + O2 > 2H20

2 : 1 2

From this, we can see that 2 moles H2 react with 1 mole of O2 to produce 2 moles of H2O. That is to say, hydrogen reacts with oxygen in the ratio 2:1

Also if 1 mole of H2 is needed to produce water, it will require 1/2 mole of O2 to react with.

Since we know this, we can tell from our question that 1 mole of H2 will react with 1/2 mole of O2 and be left with 1/2 mole of O2 as excess.

We can have a chemical equation for this as:

1/2H2 + O1/2 > 1/2H2O

1 : 1/2

Therefore, the H2 can be said to be the limiting reactant (gets consumed when the reaction is completed) while the oxygen is the excess reactant.