5 " mL of " a gas A containing only C and H was mixed with an excess of `O_2` (30 mL) and the mixture was exploded by means of electric sperk. After explosion, the remaining volume of the mixed gases was 25 mL. On adding a concentrated solution of KOH, the volume further diminished to 15 mL. The residual gas was pure oxygen, The molecular formula of the gas A is.

Explanation

To determine the molecular formula of gas A, which contains only carbon (C) and hydrogen (H), we can follow these steps:

### Step 1: Determine the volume of oxygen used
Given:
- Volume of O2 added = 30 mL
- Volume of unused O2 after the reaction = 15 mL

To find the volume of O2 used:
...