Answer:
Number of moles or volume
Step-by-step explanation:
The number 2 on the right hand side signifies the number of moles of the product being formed.
It is necessary to include this number so as not to flout the law of conservation of mass when writing a chemical equation. The law of conservation of mass states that "matter is neither created nor destroyed during the course of a chemical reaction".
Since we must have equal number of matter on both sides of the expression, the coefficient number like the 2 seen in the expression is used to create a balance.
Here, 2 moles of HCl is formed which consists of 1 hydrogen atom and 1 chlorine atom.