Question

ratio by mass of carbon and oxygen in CO2 molecule will be ?

Answer 1

Elements consist of atoms that combine in predictable ways to form compounds. When observing chemical reactions or studying chemical properties, it's sometimes important to know how to compute mass ratio -- the ratio of atoms in each of a compound's elements. You can do this once you know a couple of important properties a compound's elements possess.

Identify the elements that make up your compound. For instance, when you work with water, that compound's elements are hydrogen and oxygen. If the compound is carbon dioxide, the elements are carbon and oxygen.

Answer 2

mass of C = 12 u.
mass of 1 oxygen atom = 16 u.

As there are 2 atoms of O so the mass of oxygen here would be 2× 16 u.

ratio of carbon to oxygen in CO2

= 12 : (2 ×16)
= 3 :8