Always remember that a compound can be separated into simpler substances by chemical methods/reactions. While elements cannot be broken down into simpler substances by chemical reactions. You can do a flame test and spectrum analysis to determine whether a solid material is an element or a compound. Check the boiling and/or melting point, color or density. Also check the solid material’s reaction with oxygen, hydrogen, calcium, or various acids. Examine and study its physical chemistry. The element(s) that may be present may be identified by checking the absorption edges from an x-ray spectrum.
I'm pretty sure it's A, but then again I'm only in 8th grade. Sorry if this doesn't help:(
Answer:
1.40 moles.
Explanation:
The balanced chemical equation for the reaction of Phosphorus and Oxygen is as follows -
In Part A, the oxygen was taken in excess. So Phosphorus will be the limiting reagent.
Since, 2 moles of 
is formed by 4 moles of 
So, for 1.8 moles of amount of required moles of = 
In Part B, the phosphorus was taken in excess so oxygen will be the limiting reagent.
Since, 2 moles of is formed by 5 moles of oxygen
So, for 1.40 moles of moles of 
required = 
Thus as of now we have 3.60 moles of and 3.50 moles of .
As in the reaction of formation of , oxygen is the limiting reagent.
So the moles of formed by the 3.50 moles of oxygen will be
= 
= 1.40 moles.
