Question

How to calculate the ratio by mass of any compound?

Answer 1

Mass ratio often called percent composition by mass is the proportion of a particular molecule that consists of each of that molecule's constituent elements.
A given compound always has the same proportion by mass of its constituent elements. For instance, water always has exactly 8 grams of oxygen for every gram of hydrogen.
$H_{2}O$
Mass of H2H_2$H_2$= $2\times 1=2g$
Mass of Oxygen =16g
Ratio by mass of water
2: 16
1: 8