Finding Percent Composition From Molecular Formulas

General Chemistry Index

Where are we going with this? This page will assist in developing the ability to perform calculations to determine the composition of a compound or mixture when given the necessary information and to apply lab data to determine the empirical and molecular formula of a compound.

Finding Percent Composition From Molecular Formulas
Well, now… That's a whole lot of words!

We probably should start off with some definitions and examples, eh?

Molecular Formula: The integer ratio of elements in a compound that represents the numbers of each type of atom in the compound.

https://www.google.com/search?q=chemistry+molecular+formula

This is not new!

Working with molecular formulas goes way back. We should have a good sense about this.

So, what about that percent composition, then? What's that?

Percent Composition: The percent of the mass of a compound made up by the mass of each of the constituent elements.

https://www.google.com/search?q=chemistry+percent+composition

Another way to look at it is to think of it this way… What percent of the mass of XXXX is made up by YYYYY? What percent of water is made up by oxygen? What percent of sucrose is made up by carbon?

We can make it look more like chemistry this way…

Find the percent composition of each element in CuBr2.

So, percent… in this case, you'll be doing 

Part ÷ Whole X 100%          <--------  Look at the bottom of this page for more explaining!

Using the masses from a periodic table:

Cu = 63.5

Br = 79.9

we find the molecular mass (the whole) of CuBr2

CuBr2 = 63.6 + (2 X 79.9) 

CuBr2 = 223.3

Now, we have everything we need! Recap? Sure?

Cu = 63.5

Br = 79.9

CuBr2 = 223.3

What percent is the Cu? Let's do that Part ÷ Whole X 100% thing.

Percent Cu = 63.5 / 223.3 X 100%
Percent Cu = 28.44%

What percent is the Br? Let's do that Part ÷ Whole X 100% thing. BUT!! 

There are TWO Br in the molecule. So, the mass of the Br is not 79.9, but rather 159.8. 

Percent Br = 159.8 / 223.3 X 100%
Percent Br = 71.56%

Let's try another, but follow some steps… Steps are good, right?

Step 1: Find all the masses of the elements.

Step 2: Find the mass of the compound.

Step 3: Find the "part" masses of the compound.

Step 4: Do that Part ÷ Whole X 100% thing

Find the percent composition of each element in C2H4O2.

Step 1:

C = 12.01

H = 1.01

O = 15.99

Step 2:

C2H4O2 = 60.04

Step 3

C2 = 24.02

H4 = 4.04

O2 = 31.98

Step 4:

Percent C = 24.02 / 60.04 X 100%
Percent C = 40.01 %

Percent H = 4.04 / 60.04 X 100%
Percent H = 6.73%

Percent O = 31.98/60.04 X 100%
Percent O = 53.26%

Not so bad! That's it. 

https://youtu.be/iEpkBXfPFto

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What is this Part ÷ Whole X 100% thing?

Math… If you want to know what percent of something you have, you… do that.

A classroom has 10 red desks and 30 blue desks. What percent of the desks are red?

The "part" you are looking at is the red desks. The "whole" is ALL of the desks.

Percent red desks =  Part ÷ Whole X 100%

Percent red desks = 10 ÷ 40 X 100%

Percent red desks = .25 X 100%

Percent red desks = 25%

It works like this for all "what percent of" questions. Math…