Tuesday, November 30, 2010

Percent Composition

So what did we learn today? That's right! Percent Composition!!!
Percent Composition is the mass percentage of a element in a molecule. 

To calculate percent composition it's as easy as 1 2 3! There's no formula to memorize... you wanna know the secret to it? 
There's no secret! In fact you do it all the time to calculate the percent for your marks. But if you do insist a formula here it is: % Composition = mass of element      X 100%
                                                            mass of compound  
Now you know this not so  secret formula, let's try it out!

EX 1. Calculate % composition of C2H4O2
 Total MM = 60 g/mol (the mass of C2H4O2)

% of C = 24 g/mol X 100 = 40%
                60 g/mol
           
% of H = 4 g/mol X 100% = 6.7%
               60 g/mol

% of O = 32 g/mol X 100% = 53.3%
                60 g/mol

And that's it! It's that simple! Bet you can't wait to do more of these eh? Well, you're in luck here's a few more examples!

1) Calculate the % composition of FeO.
2) Calculate the % composition of H2O
3)Calculate the % composition of Co(NO3)2
4)What is the % composition of the underlined portion of this compound C8H9NO2
5) A compound has a mass of 51.2 g. It contains 20g of O, 19.2 g of S and a mystery amount of N. What is the % composition of N?

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ANSWERS AND SOLUTIONS:
1) Total MM = 71.8 g/mol
% of Fe = 55.8 g/mol / 71.8 g/mol = 77.7%
% of O = 16 g/mol / 71.8g/mol = 22.3%
2) Total MM = 18 g/mol
% of H = 2g/mol / 18g/mol = 11.1%
% of O = 16.0g/mol / 18g/mol =88.9% 
3) Total MM = 156.9 g/mol
% of NO3 = 124 g/mol / 156.9g/mol = 79.0%
% of Co = 58.9g/mol / 156.9g/mol = 37.5%
4) Total MM = 151g/mol
% of C = 96g/mol / 151g/mol = 63.6%
% of H = 9g/mol / 151g/mol = 5.96%
% of N = 14.0g/mol / 151g/mol = 9.27%
% of O = 32g/mol / 151g/mol = 21.1%
5)51.2- 20-19.2= 12 g of N 
Total MM = 51.2 g/mol
% of N = 12g/mol / 51.2g/mol = 23.4%






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