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The statement which best describes the law of conservation of mass is A) when a physical or a chemical change occurs, matter is not created or destroyed. The law states that matter cannot be created or destroyed by ordinary chemical or physical changes, which means that <span>the mass of all the components of a chemical reaction can be measured before and after the change in order prove that the mass is constant. So, keep in mind that </span><span>the mass of participating products is always the same as the mass of all the reactants.</span>
Answer:
Excess Reagent = oxygen
Explanation:
Limiting reagent: The substance that is totally consumed when the reaction is completed.
Excess reagent: The substance left after the limiting reagent is consumed completely
The balanced chemical equation for formation of water is as follow:

This means when 2 moles of hydrogen reacts with 1 mole of oxygen, 2 moles of water is produced.
Hence the ratio in which hydrogen and oxygen gas reacts is 2:1
Now if 2 mole hydrogen require 1 mole of oxygen ,then 4 mole hydrogen need 2 mole of oxygen.

or

Here 5 mole of oxygen is reacting but only 2 mole is required .
Oxygen is in excess.