How to Find the Percent Composition by Mass for a Compound

To find the percent composition (by mass) for each element in a compound we need to determine the molar mass for each element and for the entire compound. We’ll use the following formula for percent composition by mass:

Percent Composition by Mass = (Molar Mass of Element x Number of that Element)/ Molar Mass of Compound

In this video I’m using a Periodic Table that rounds to two decimal places. You may be asked to use a Periodic Table with more decimal places, however the process to find the percent composition by mass is the same.

Molar Mass in Three Easy Steps: youtu.be/o3MMBO8WxjY

For help with Stoichiometry (moles, grams, liters, mole ratios ...):

• More Moles to Grams Practice: youtu.be/aIv5nr8ZNyw
• Molar Mass in Three Easy Steps: youtu.be/o3MMBO8WxjY
• Understanding the Mole: youtu.be/DyLktMPTuHY
• Moles - Gram Conversions: youtu.be/aIv5nr8ZNyw
• How to Balance Chemical Equations: youtu.be/zmdxMlb88Fs
• Mole Ratio: youtu.be/i71BMVlrMiw
• Reaction Stoichiometry: youtu.be/rrTqOsZPpaU
My chemistry website: Breslyn.org

Converting between moles and grams is the cornerstone of being successful in stoichiometry, the study of chemical quantities. Take the time to learn mole conversions and you will find chemistry is much easier.

The use of conversion factors (also called factor-label method or dimensional analysis) is a more general technique for converting quantities. Once you understand how it works it can be applied to many different conversion (as long as you know the conversion factor).