Question

What is the percent composition of a carbon, in heptane, ${{C}_{7}}{{H}_{10}}$ ?

Answer 1

Hint: The percentage of an atom in a molecule can be calculated by using the below formula.
The percentage of an atom in a molecule = $\dfrac{W}{M.W}$
W = molecular weight of the particular atom
M.W = Molecular weight of the total molecule.

Complete answer:
- In the question it is asked to calculate the percent composition of a carbon, in heptane.
- The molecular formula heptane is ${{C}_{7}}{{H}_{16}}$ .
- The total molecular weight of the compound (heptane) can be calculated as follows.
- Heptane contains seven carbon atoms and 16 hydrogen atoms.
- The molecular weight of each carbon atom is 12 and the molecular weight of each hydrogen atom is 1.
- The total molecular weight of the compound is = (12) (7) + ( 16) (1) = 100.
- Now by using the total molecular weight of the heptane we can calculate the percentage of carbon which is present in the heptane and it is as follows.
The percentage of an atom in a molecule = $\dfrac{W}{M.W}$
W = molecular weight of the carbon atom = (12) (7) = 84
M.W = Molecular weight of the total molecule = 100
The percentage of carbon in heptane = $\dfrac{W}{M.W}=\dfrac{84}{100}=84%$
- Therefore the percentage of carbon in the heptane is 84 %.

Note:
We should know the molecular weight of the individual atoms involved in the molecular formula to calculate the percentage of a particular atom and we have to calculate the total molecular weight of the molecule too.