Percent composition is the percentage by mass of a "species" in a chemical formula.
Before I tell you how to calculate percent composition, check out this video! It should help out a bit.
To calculate the percent composition of a compound
- Calculate the molecular mass (molecular weight, formula mass, formula weight), MM, of the compound
- Calculate the total mass of each element present in the formula of the compound
- Calculate the percent compositon (percentage composition): % by weight (mass) of element
= (total mass of element present ÷ molecular mass) x 100
After you calculate the percentage of each compound, all of the percentages should add up to 100.
1. Calculate the composition of C2H5OH.
- Calculate the Molar Mass of this object.
- Carbon has a mass of 12.0g. There are 2 atoms of Carbon in this substance, therefore the mass of carbon in this substance is 24.0g/mol.
- Hydrogen has a mass of 1.0g. There are 6 atoms of Hydrogen in this substance, therefore the mass of hydrogen in this substance is 6.0g/mol.
- Oxygen has a mass of 16.0g. There is one atom of Oxygen in this substance, therefore the mass of oxygen in this substance is 16.0g/mol.
- Add it all together to calculate the Molar Mass: 46g/mol.
- % of Carbon: 24.0g / 46.0g. This is the mass of carbon divided by the total Molar Mass. Then multiply it by 100. = 52.2%.
- % of Hydrogen: 6.0g / 46.0g. Multiplied by 100 = 13%.
- % of Oxygen: 16.0g / 46.0g. Multiplied by 100 = 34.8%.
2. If a compound contains 8.5g of F, 19.0g of Mg, and some amount of Be, and has a total mass of 41, calculate the % composition.
This question is harder because we have to figure out what the amount of Be is.
- The total MM is 41g. So add 8.5g and 19.0g to get 27.5. 41-27.5 = 13.5g of Be.
- MM of Fluoride: 8.5g
- MM of Magnesium: 19.0g
- MM of Beryllium: 13.5g
- % of F: 8.5g / 41g x 100 = 20.7%
- % of Mg: 19.0g / 41g x 100 = 46.3%
- % of Be: 13.5g / 41g x 100 = 33%
Pretty simple, right?
Just in case it still doesn't make sense, here is a step-by-step tutorial of an "Easy" example of Percent Composition.
If you're just breezing through this, here is a step-by-step tutorial of a "Harder" example of Percent Composition - made by the same guy!