The decomposition of ammonia is characterized by the following decomposition equation:
2NH₃<span> → N</span>₂ <span> + 3H</span>₂
The mole ratio of N₂ : H₂ is 1 : 3
If the number of moles of N₂ = 0.0351 mol
Then the number of moles of H₂ = 0.0351 mol × 3
= 0.1053 mol
The number of moles of hydrogen gas produced when 0.0351 mol of Nitrogen gas is produced after the decomposition of Ammonia is 0.105 mol (OPTION 3).
Answer: 2. cannot be an element.
Explanation:
Element is a pure substance which is composed of atoms of similar elements.It can not be decomposed into simpler constituents using chemical reactions.Example: Copper 
Compound is a pure substance which is made from atoms of different elements combined together in a fixed ratio by mass.It can be decomposed into simpler constituents using chemical reactions. Example: methane 

Thus as it can be decomposed into its constituent elements, it is a compound.
Mixture is a substance which has two or more components which do not combine chemically and do not have any fixed ratio in which they are present.
Thus methane cannot be an element.
Not strong base and acid, not dissolved or not aqueous.