Number of oxygen atoms present in chemical formula of fuming sulphuric acid is ______.

We need to find the number of oxygen atoms in the chemical formula of fuming sulphuric acid.

Fuming sulphuric acid, also known as oleum, is a solution of sulphur trioxide ($$SO_3$$) in concentrated sulphuric acid ($$H_2SO_4$$). The chemical formula of oleum, also called pyrosulphuric acid or disulphuric acid, is:

$$H_2S_2O_7$$

It can be thought of as the combination of $$H_2SO_4$$ and $$SO_3$$:

$$H_2SO_4 + SO_3 \rightarrow H_2S_2O_7$$

Counting the atoms in $$H_2S_2O_7$$ gives 2 hydrogen atoms, 2 sulphur atoms, and 7 oxygen atoms. Therefore, the number of oxygen atoms in the chemical formula of fuming sulphuric acid is 7.