Answer:
c. an element.
Explanation:
An element -
It refers to the substance , which has same type of atoms , with exactly same number of protons , is referred to as an element .
In term of chemical species , elements are the smallest one , and can not be bifurcated down to any further small substance by the means of any chemical reaction .
Hence , from the given information of the question ,
The correct term is an element .
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Due to the high strength of iron compared to that of barium and phosphorus the nails are made of iron.
Though iron oxidizes to form rust on reacting with water but it does not explode. Whereas barium reacts vigorously with water with evolution of hydrogen gas. Barium on reacting with other nonmetals results in the formation of poisonous substances. Phosphorus is never found in its pure form in nature and is combustible.
Hence, nails are made up of iron but barium and phosphorus are not suitable for making of nails.
One kilogram is the answer
We know that
1) empirical formula of a compound is the representation of a molecules with the elements in simplest mole ratio.
2) the molar mass is the actual proportion of moles of elements present in a molecule
3) there is a simple ratio between the molar mass and empirical mass a molecule
Now let us solve each problem
1) empirical formula of 
will be 
2) empirical formula of 
will be
3) empirical formula of 
will be 
4) the molecular formula will be :
the molecular formula = 4 X empirical formula = 4 X CO = 
5) the mole ratio of Cl and Cr is 
empirical formula will be 
