Percent Composition

What is Percent Composition?  The percent composition by mass of a compound represents the percent that each element in a compound contributes to the total mass of the compound.

How do I calculate percent composition?  Formula:  %mass = grams of element in 1 mol compound x 100% molar mass of compound Example: What is the percent composition of water (H 2 O)? Step 1: figure out the molar mass from the formula 2 mol Hydrogen x 1.01g = 2.02g 1mol Oxygen x 16.00g = 16.00g molar mass of water = 18.02g Step 2: divide the answer for each atom by the molar mass and multiply by 100 to get a percentage 2.02 g of H / g of H 2 O x 100 % = 11.21% Hydrogen g of O / g of H 2 O x 100 % = 88.79% Oxygen Your percentages should add up to 100%

Same concept, but worded differently  An 8.20g piece of magnesium combines completely with 5.40g of oxygen to form magnesium oxide. What is the percent composition of the magnesium oxide?  Solution: Step 1: Find the mass of the compound 8.20g Mg g O = g Step 2: Find the percent by mass of each element %Mg = 8.20g x 100% = 60.29% of Mg g %O = 5.40g x 100% = % of O g

Your Turn:  What is the percent composition of glucose (C 6 H 12 O 6 )  Step 1: figure out the molar mass:  Step 2: Divide the answer of each atom by the molar mass and multiply by 100

Practice Problems  Calculate the percentage composition by mass of the following compounds  1) HgO  2) Na 2 S  3) (NH 4 ) 2 S  4) Sr(NO 3 ) 2