In chemistry, mass ratio, often called "percent composition by mass," is the proportion of a particular molecule that consists of each that molecule's constituent elements. For example, water consists of 11.1 percent hydrogen (H) and 88.9 percent oxygen (O), meaning that a 1,000-gram sample of water (equal to 1 liter in volume) consists of 111 g of H (0.111 × 1,000 = 111) and 889 g of O (0.889 × 1,000).
This principle gives rise to the Law of Constant Composition, put forth by Joseph Proust in 1800: 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. Carbon dioxide always has 2.67 g of oxygen for every gram of carbon.
Calculating mass ratios is easy enough if you have access to a periodic table (see Resources) and the means to do basic algebra.
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Say you want to calculate the mass ratio of sulfuric acid, H2SO4.
Determine the Molar Mass of Each Element Present
H2SO4 contains hydrogen (H), sulfur (S) and oxygen (S). From the periodic table, you can see that the molar masses of these elements are:
H = 1.00
S = 32.06
O = 16.00
Step 2: Determine the Mass of Each Individual Element Present
In this step, you multiply the number of atoms in one molecule of the compound by the molar masses you collected in Step 1. The number of atoms is simply the subscript after the element's abbreviation in the molecular formula, with the omission of a subscript signifying "1."
There are two H atoms present, one S atom and four O atoms, so you have:
H = (2)(1.00) = 2 g
S = (1)(32.06 g) = 32.06 g
O = (4)(16.00 g) = 64 g
Step 3: Determine the Molar Mass of the Compound
Add together the figures you calculated in Step 2:
2 + 32.06 + 64 = 98.06 g
Step 4: Divide the Mass of Each Element Present by the Molar Mass
This means dividing the individual masses from Step 2 by the result of Step 3.
For H, you have 2 ÷ 98.06 = 0.0204 = 2.04 percent hydrogen
For S, you have 32.06 ÷ 98.06 = 0.3269 = 32.69 percent sulfur
For O, you have 64 ÷ 98.06 = 0.6527 = 65.27 percent oxygen
To check your work, make sure your percentages sum to 100, allowing for tiny differences owing to rounding:
2.04 + 32.69 + 65.27 = 100.0