How to – Percent Composition

In compounds and formulas in chemistry, we must understand that the parts of a compound are not all of equal value as far as being parts of the whole. This is what we call percent composition, or the amount something is out of the whole (% value out of 100%). Finding the percent value of an element in a compound is not very difficult, and requires only basic math and a little information.


1. The first step in finding percent composition is to find the mass of each of our components. For this example, we will use the simple compound Copper(II) Sulfate. Through research, we determine that copper (II) has a mass of 63.5g, sulfur has a mass of 32.1g, and oxygen has a mass of 16.0g. Now, notice the (4) next to the 16.0g for oxygen. This is due to the subscript for oxygen. The subscript tells us that there are four components of oxygen, meaning in our calculations, we must account for this and multiply the mass of oxygen by for.


2. The second step is short and sweet. Simply, we are adding our component values to find the mass of our substance. So, we take the 63.5g of copper(II), the 32.1g of sulfur, and the 64.0g of oxygen(16.0g multiplied by the subscript of 4) and add them together. In all, we find the mass of our substance to be 159.6g.


3. The last step is not difficult at all, but will perhaps take the most time without a calculator. In this step, we want to use our data to find the percent composition of our substance. To keep this simple, we will just be looking for the percent of copper(II) in our compound. To do this, we must divide the part(mass of Cu(II)) by the whole(total mass of CuSO4). Then, we take our answer and multiply it by One-hundred. So, we would take 63.5g and divide it by 159.6g, and then take that answer and multiply it by 100. This would give us a percent of 39.7869. However, we must remember to user significant digits, and with our calculations, the least amount of significant digits used was three. So, our final percent of copper(II) came out to 39.8%.
So in essence, all that is left is to repeat step three with each component, giving you the percent composition of our compound.

Hopefully this step-by-step guide has helped you to better understand the concept of percent composition, as well as how to calculate it.


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