Percentage Composition

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  • What is percentage composition? It is the percentage(s) of elements in a compound. Let's practice with CH4.
  • The first thing we need to find out is the molecular weight. Basically, find the atomic weight of each element and add them together. C has an atomic weight of 12.01 amu, and Hydrogen has an atomic weight of 1.008 amu. Now add, 12.01+ 4(1.008). Our molecular weight is 16.042
  • After we find the molecular weight, we calculate the percentage of each element in the compound. Just multiply the number of atoms by its own atomic weight. Then divide that number by the molecular weight. Finally, multiply the number by 100%.
  • Let's apply that information. Carbon only has 1 atom in this compound. So 12.01 divided by 16.042 is 0.7486. Now we multiply by 100%, which will give us 74.86%. Round 74.86% to 75%. However, hydrogen has 4 atoms in this compound. So multiply 1.008 by 4, and you should end up with 4.032. Then divide by 16.042, and you will get 0.2513. Then, multiply this number by 100%, which will be 25.13%. Finally, round 25.13% to 25%.
  • Checking your calculations is very simple. Just add your element percentages together. The sum should be 100, if not something is wrong. 75% of C + 25% of H = 100%
  • We know this is correct, since both percentages add up to 100%.
  • Based off of our calculations, 75% of carbon and 25% of hydrogen makes up CH4.
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