How to Calculate Percentage Composition by Mass

Let's learn how to calculate percentage composition by mass in this video.

The percentage mass of an element X in a compound is equal to the percentage molar mass of X in that compound.

230213 percentage mass formula

This means the mass ratio of X to compound is equal to molar mass ratio of X to compound.

To understand this, we can use water as an example.

230213 water

One molecule of water is made up of 2 hydrogen atoms and 1 oxygen atom.

Since water is a compound made up of 2 or more elements chemically combined together in a fixed proportion, the mole ratio of hydrogen to oxygen will always be 2 to 1 regardless of the amount of water.

In 1 gram of water, the mole ratio of H to O is 2 to 1, and we can determine the mass ratio of H to O.

In 1kg of water, the mole ratio of H to O is still 2 to 1, therefore the mass ratio of H to O will remain the same.

In fact in any quantity of water, the mass ratio of H to O will be constant since mole ratio of H to O will always be 2 to 1.

We can peg this mass ratio to the molar mass ratio since we can determine molar mass from the periodic table.

For example to determine mass of oxygen in 10g of water, we can use the mass ratio equal to molar mass ratio formula to get our answer quickly.

230213 find mass of O in 10g water

Topic: Mole Concept, Physical Chemistry, A Level Chemistry, Singapore

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