Question

The formula of Iron (lll) sulphate is _______.

Answer 1

For the writing formula of any compound, we simply cross multiply their respective elements. For example, formula of sodium chloride. Here, the valency of sodium (Na) is $+1$ and the valency of chloride (Cl) is $-1$. So, after cross multiplying of valency it becomes $N{{a}_{1}}C{{l}_{1}}$ or simply $NaCl$.

Complete answer:
-Here the compound is Iron (lll) sulphate. The valency of iron is +3 and sulphate $(S{{O}_{4}})$ is -2. therefore, after cross multiplication it becomes $F{{e}_{2}}{{(S{{O}_{4}})}_{3}}$.
$F{{e}_{2}}{{(S{{O}_{4}})}_{3}}$ is commonly known as ferric sulphate. Ferric sulphate is an inorganic compound which appears as a yellow crystalline solid or a greyish powder.
-The molecular weight of ferric sulphate $F{{e}_{2}}{{(S{{O}_{4}})}_{3}}\,\,=\,\,2\times 56+3(32+4\times 16)=112+3(96)\,=\,112+288\,=400\,g/mol$.
-It is the sulphur of trivalent iron. Usually, it is of dark brown or yellow colour. It is rhombic crystalline salt and soluble in water at room temperature.
-It is acidic in nature. It is produced by reaction of sulphuric acid and oxidising in nature.
-Let's find the number of atoms in ferric sulphate. So, let’s learn how to calculate the number of atoms in a particular compound. To find the total number of atoms $F{{e}_{2}}{{(S{{O}_{4}})}_{3}}$ Iron(lll) sulphate. We will add up the number of each type of atom. The small number after the element symbol is called the subscript and this tells us how many of that atom are in the compound. For example, ${{H}_{2}}$ means there are two hydrogen atoms.
-When there is no number after an element we assume there to be one atom of that element. For example, in ${{H}_{2}}O$ we have only one oxygen atom. If there is a number in front of the compound it is called coefficient. Like $2{{H}_{2}}O$ that means we have two separate molecules. We therefore multiply the subscript for each element by the number in front. For $2{{H}_{2}}O$ we have four H atoms and two O atoms.
-For compounds with parentheses like $Ca{{(N{{O}_{3}})}_{2}}$, we multiply the atom inside the parenthesis by subscript.
-The number of atoms in $F{{e}_{2}}{{(S{{O}_{4}})}_{3}}$ is 2 iron, 3 sulphur and 12 oxygen.
-The number of iron atoms in 1 mole of ferric sulphate is $2\times {{N}_{A}}$ . Similarly, the number of sulphur and oxygen atom in 1 mole of ferric sulphate is $3\times {{N}_{A}}$ and $12\times {{N}_{A}}$ respectively.

Note:
Ferric sulphate is used in dyeing as a mordant and as a coagulant for industrial wastes. It is also used for water purification and as a soil conditioner.