Mass Percent Composition

Problem

What are the mass % of carbon and oxygen in carbon dioxide, CO₂?

Solution

First, look up the atomic masses for carbon and oxygen from the Periodic Table. The atomic masses are found to be:

C is 12.01
O is 16.00

Next, determine how many grams of each element are present in one mole of CO₂:

12.01 g (1 mol) of C
32.00 g (2 mole x 16.00 gram per mole) of O

The mass of one mole of CO₂ is:

12.01 g + 32.00 g = 44.01 g

And the mass percentages of the elements are

mass % C = 12.01 g / 44.01 g x 100 = 27.29 %
mass % O = 32.00 g / 44.01 g x 100 = 72.71 %

Answer

mass % C = 27.29 %
mass % O = 72.71 %

When doing mass percent calculations, it is always a good idea to check to make sure your mass percents add up to 100% (helps catch math errors):

27.29 + 72.71 = 100.00

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