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Composition By Percentage

The MCAT often contains percentage composition problems. Percentage composition is the percentage contribution (by weight) of each element to the total mass. Let's explore this idea by looking at some examples.


Calcium carbonate (CaCO3), commonly known as limestone is used in the preparation of a variety of compounds. Calculate the percentage composition of each element in calcium carbonate.



# of atoms per molecule molar weight of the atoms total mass of the element per mol
1 Calcium 1 x 40.1 g 40.1 g
1 Carbon 1 x 12.0 g 12.0 g
3 Oxygen 3 x 16.0 g 48.0 g
    100.1 g
The percentage composition of each element can be found as follows:


Predicting Formulas from Percentage Compositions

You should be able to predict the formula of a compound on the basis of a given data of percentage compositions. Study the next example to understand how it is done.

A carbon compound contains 27.27% carbon and 72.73% oxygen by mass. Predict the simplest ratio or formula of the compound.



The best way to approach this problem is to consider that we have 100 grams of this compound. Logically it should contain 27.27 grams of carbon and 72.73 grams of oxygen. With that in mind, we can calculate the number of moles of each element or atom. After that we can obtain the simple ratio.


Step 1
# of moles of carbon atoms equals 27.27/12 = 2.275 moles of carbon atoms
# of moles of oxygen atoms equals 72.73/16 = 4.546 moles of oxygen atoms

Step 2
Divide every number of moles with the smallest number of moles that you got in Step 1. Here the smaller one is 2.2725. So divide the number of moles of carbon atoms and the number of moles of oxygen atoms by 2.2725. That will give you the simplest ratio between them.
Carbon: 2.2725/2.2725 = 1
Oxygen: 4.546/2.2725 ≈ 2
Since the ratio of carbon to oxygen is 1:2, the compound is CO2.

Calculate the mass of sulfur in 150 grams of H2SO4.


The easiest way to calculate this is to find the percentage composition of sulfur. Then, use that percentage to find the mass of sulfur in the given amount of substance.

Step 1
% of sulfur = 32.1/98 x 100, roughly 33%

Step 2
The mass of sulfur present in 150 grams of sulfuric acid is
150 x 33%/100 = 49.5 g

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