Question

Calculate the percentage composition of iron in ferric oxide, $F{{e}_{2}}{{O}_{3}}$ (atomic mass of Fe = 56)
A. 70%
B. 56%
C. 87%
D. 35%

Answer 1

Hint: We have to follow the below formula to calculate the percentage of any ion present in the combined chemical form.
\[\text{ percentage of metal in the chemical = }\dfrac{\text{total mass of the metal}}{\text{total mass of the compound}}\times \text{ 100}\]

Complete answer:
Molecular formula of ferric oxide is $F{{e}_{2}}{{O}_{3}}$.
Initially we have to calculate the total mass of the ferric oxide.
Ferric oxide has two iron atoms and three oxygen atoms.
Molecular weight of oxygen = 16
Molecular weight of iron = 56
\[\begin{align}
  & \text{Total mass of oxygen=3 }\!\!\times\!\!\text{ 16} \\
 & \text{ =48 grams of oxygen} \\
 & \text{Total mass of iron=2 }\!\!\times\!\!\text{ 56} \\
 & \text{ =112 grams of iron} \\
\end{align}\]
Adding the mass of total oxygen and total iron gives the molecular weight of the ferric oxide.
     Total mass of ferric oxide = 48+ 112 =160 grams
Percentage of an atom in the chemical
\[\begin{align}
  & \text{ percentage of iron in the chemical = }\dfrac{\text{total mass of the iron}}{\text{total mass of the ferric oxide}}\text{ }\times \text{ 100} \\
 & \text{=}\dfrac{\text{112}}{\text{160}}\text{ }\times \text{ 100} \\
 & \text{=70 }\text{% } \\
\end{align}\]
The percentage of iron in ferric oxide is 70%.

So, the correct option is A.

Additional information:
Both ferrous oxide and ferric oxide are in-organic compounds.
Ferric oxide is a more stable form of iron than the ferrous oxide.
Ferric oxide $F{{e}_{2}}{{O}_{3}}$ is a reddish-brown powder.
Ferrous oxide $FeO$ is a black-colored powder.

Note:
Don’t be confused with the words ferrous oxide and ferric oxide:
The molecular formula of Ferrous oxide- $FeO$
The molecular formula of Ferric oxide- $F{{e}_{2}}{{O}_{3}}$
In ferrous oxide $FeO$ the oxidation state of iron is +2.
In ferric oxide $F{{e}_{2}}{{O}_{3}}$ the oxidation state of iron is +3.