Question

What does the formula of a substance tell you?

Answer 1

The chemical formula is another name for the molecular formula of a compound. The information contained in a chemical formula is based on the certain rules known as the laws of chemical combination.

Complete answer:
The chemical formula of a substance simply contains the chemical symbols of all its constituent elements.
The law of definite proportions suggests that the formation of any compound will always happen in fixed proportion or ratios. Unlike simple homogeneous mixtures, the elements of a compound always combine with each other through strong bonds and in definite proportions by mass.
Thus the chemical formula contains the information regarding all the types of elements responsible for making up a molecule and the subscript on the symbol of each element represents the stoichiometric number of that element i.e. the number of atoms of that element that are present per molecule of the compound.
For example: The chemical formula of water is ${H_2}O$ , which tells us that it consists of two elements namely hydrogen and oxygen. We also get to know that hydrogen and oxygen always combine in the ratio of $2:1$ i.e. one molecule of water will always contain two atoms of hydrogen for every one atom of oxygen.

Note:
A substance can be an element in its pure form i.e. in its most stable state of aggregation or it can be a compound made up of two or more elements combined together through chemical bonds in certain specific proportions.