December 15th - Percent Composition

And just before we relax, we had one more lesson to cover. It was a fairly quick one, and was all about percent composition. Though it is common to think that we were talking about the percentage of atoms in a compound, we were not! In fact, we were exploring the percent composition by mass.

Well, the best way to learn this is by doing an example. Let’s start off with the most common solvent, water. Water, with a chemical formula H₂O, has a molar mass at 18.0 g/mol. The hydrogen accounts for 2.0 g/mol, while the oxygen makes up the remaining 16.0 g/mol. Now, we can simply make a ratio of the element’s molar mass to the total.

Hydrogen: 2/18 = 11%

Oxygen: 16/18 = 89%

And there you have it! It’s as simple as that.

Let’s try percent composition with a twist. Let’s try to find the empirical formula of a substance. 


Ex. 1) An unknown substance is made up of:

·         2.04% H

·         65.3% O

·         32.65% S

Let’s assume we have 100g to work with. Therefore, we have

·         2.04g of H

·         65.3g of O

·         32.65 g of S

Now we need to find out how many moles of each substance we have.

·         Hydrogen = 1.0 g/mol = 2.04mol

·         Oxygen = 16.0g/mol = 4.08mol

·         Sulphur = 32g/mol = 1.02 mol

We can now make our empirical formula. Roughly, the ratio is H₂SO₄. And there you have it; sulphuric acid. Here's a video further explaining:

Posted by Andrew.