Atom is the
most basic unit of matter. They are the general term used to describe pieces of
matter. But a different kind of atom makes up an element. A combination of atom of different elements
creates a molecule. For example, you have water; it has a chemical formula of
H2O. If you separate H from O, you have two atoms. Two atoms for hydrogen and
one atom for oxygen. However, they are of different elements. And when you
combine them to form H2O, you create a molecule.
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Answer:
The correct answer is A :))
Answer : The reagent present in excess and remains unreacted is, 
Solution : Given,
Moles of
= 3.00 mole
Moles of
= 2.00 mole
Excess reagent : It is defined as the reactants not completely used up in the reaction.
Limiting reagent : It is defined as the reactants completely used up in the reaction.
Now we have to calculate the limiting and excess reagent.
The balanced chemical reaction is,

From the balanced reaction we conclude that
As, 2 moles of
react with 1 mole of 
So, 3.00 moles of
react with
moles of 
From this we conclude that,
is an excess reagent because the given moles are greater than the required moles and
is a limiting reagent and it limits the formation of product.
Hence, the reagent present in excess and remains unreacted is, 