Percent Composition

 What is percent composition? Well... it is a compound that is a relative measure of the mass, of the different elements that are in the compound. Anyways...you need to be able to figure out the molar mass of each element, so if you are not sure what the molar mass of elements are, please read the previous blogs!!

This is an example of what a element would consist of, if the percent composition was represented by a pie graph

Now... how to actually calculate the percent composition of an element...

Ex 1/ What is the percentage composition of H20

First... you need to find:
Total Molar Mass of H2O.= 18.0g
Molar Mass H = 2.0g/ mol
Molar Mass O = 16.0/ mol

After that you find the % of each element, by dividing it by the total molar mass.

% of H = 2.0g per mol/ 18.0grams x100 = 11.1%
% of O = 16.0 per mol/ 18.0 grams x 100 = 88.9%

11.1%+88.9%= 100%

* Because there were no numbers in the questions, then you don't have to count the significant figures, so just round the number by one decimal place

Ex 2/ What is the percentage composition of Sc2(SO4)3

Total Molar Mass: 333.3 grams
Molar Mass Sc: 45.0 g/mol
Molar Mass S: 96.3 g/mol
Molar Mass O: 192.0 g/mol

% of Sc = 45.0 grams per mol / 333.3 grams x100 = 13.5%
% of S = 96.3 grams per mol / 333.3 grams x 100 = 28.9%
% of O = 192.0 grams per mol / 333.3 grams x 100 = 57.6%

The total percentage totals up to 100%

Ex 3/ What is the percentage composition of Zn(ClO3)2

Total Molar Mass: 232.4 grams
Molar Mass Zn: 65.4 grams/mol
Molar Mass Cl: 71.0 grams/mol
Molar Mass O: 96.0 grams/mol

% of Zn: 65.4 grams per mol / 232.4grams x100 = 28.1%
% of Cl: 71.0 grams per mol / 232.4 grams x100 = 30.6%
% of O: 96.0 grams per mol / 232.4 grams x 100 = 41.3%

The total adds up to 100% YAY!

Ex 4/ If a compound contains 48.0 grams of C, 6.0 grams of H, 64.0 grams of O, and contains some amount of Be and has a total mass of 127.0 grams. Calculate the % composition.

127.0-48.0-6.0-64.0 = 9.0 grams

% of C: 48.0 g per mol / 127.0 grams x 100 = 37.8%
% of H: 6.0 g per mol / 127.0 grams x 100 = 4.7%
% of O: 64.0 g per mol / 127.0 grams x 100 = 50.4%
% of Be: 9.0 g per mol / 127.0 grams x 100 = 7.1%

*Remember, there are numbers in the question, so sig figs count!

FINALLY......

Ex 5/ If a compound contains 137.3 g of Ba, 28.0 g of N, and contains some amount of O and has a total mass of 261.3 grams. Calculate the percent composition.

261.3 - 137.3 - 28.0 = 96.0 grams

% of Ba: 137.3 g per mol/ 261.3 x 100= 52.5%
% of N: 28.0 g per mol / 261.3 x 100 = 10.7%
% of O: 96.0g per mol / 261.3 x 100 = 36.7%

In the case the percentages only add up to 99.9%. Due to the rounding, there may be some cases where the percentages do not add up to 100%. If it is only 0.1% off, it is not a big deal, but to be safe ask Ms. Chen =]

These video's can maybe explain in more detail on how to calculate percent compositions..






After having all that knowledge inputted into your brain... time to use those skills on some other problems!

http://misterguch.brinkster.net/PRA023.pdf
http://cmsweb1.loudoun.k12.va.us/52820831134912597/lib/52820831134912597/Moles/Homework/masspercomp.pdf



Good Luck...