Percent Composition

Percent Composition The percentage (by mass) of each element in a compound Percent – means ‘parts per hundred’ So, you have 100 beans. If 18 beans are yellow – 18% If the rest are black, what percentage of black beans would you have? 82% black

Determining Percent Composition The problem can be presented two ways… All of the info you need is given, or You have to figure out molar masses… Either way, what you do… Divide the mass of the element by the total mass of the compound then multiply by 100. That’s the percent!

This is what it looks like when a problem gives you all the info… You have 50g of NaCl. If you have 9g of sodium, what is the % composition? Mass of element = % Total mass comp. X 100 9 g x 100 = 50 g 18% How many grams of Cl do you have? What percent is that? 41 g 82 %

This is what it looks like when you have to find the molar masses from the chemical formula… Determine the percent composition of Ammonium Sulfate, (NH4)2SO4. 1st, find the total molar mass for the compound. (2x14.01) + (8x1.01) + 32.07 + (4x16.00) = 132.17 g/mol Then, find each element’s percent comp. ( 2 x 14.01) / 132.17 x 100 = 21.2% N ( 8 x 1.01) / 132.17 x 100 = 6.1% H 32.07 / 132.17 x 100 = 24.3% S ( 4 x 16.00) / 132.17 x 100 = 48.4% O

Finding Grams from Percent You have a 90g sample of aluminum oxide that is 28% aluminum, how many grams of Al is that? The question is essentially asking, “What is 28% of 90?” First, convert 28 % to a decimal: 28% / 100  0.28 Then, 0.28 x 90 g is the amount of Al  25.2 g

Using Percent Composition You have two samples of iron oxide. Determine if they are the same compound. A 36 g sample contains 28 g Fe. A 160 g sample contains 112 g Fe. (If they have the same percent of iron, they must have the same percent of oxygen and will therefore be the same compound!) (28 / 36) x 100 = (112 / 160) x 100 = 78% 70% Not the same!!!

Using Percent Composition How many grams of oxygen can be recovered when 50 g of KMnO4 decomposes? Find the % oxygen in the compound. Molar Mass: 158.04 g/mol [(4 x 16) / 158.04] x 100 = 40.5% O 40.5% of 50 grams is how much oxygen can be recovered: 0.405 x 50 g = 20 g of oxygen