Percent CompositionPercent Composition

What is it?What is it?

The percent composition by mass of a The percent composition by mass of a compound represents the percent that compound represents the percent that each element in a compound contributes each element in a compound contributes to the total mass of the compound. to the total mass of the compound.

How does one calculate it?How does one calculate it?%mass = %mass = grams of element in 1 mol compoundgrams of element in 1 mol compound x 100% x 100%

molar mass of compoundmolar mass of compoundExample: What is the percent composition of water (HExample: What is the percent composition of water (H22O)?O)?

Step 1Step 1: figure out the molar mass from the formula: figure out the molar mass from the formula

2 mol Hydrogen x 1.01g = 2.02g2 mol Hydrogen x 1.01g = 2.02g

1mol Oxygen x 16.00g = 1mol Oxygen x 16.00g = 16.00g16.00g

molar mass of water = 18.02gmolar mass of water = 18.02g

Step 2Step 2: divide the answer for each atom by the molar mass and : divide the answer for each atom by the molar mass and

multiply by 100 to get a percentagemultiply by 100 to get a percentage 2.02 g of H / 18.02 g of H2.02 g of H / 18.02 g of H22O x 100 % = O x 100 % = 11.21% Hydrogen11.21% Hydrogen

16.00 g of O / 18.02 g of H16.00 g of O / 18.02 g of H22O x 100 % = O x 100 % = 88.79% Oxygen88.79% Oxygen

Your percentages should add up to 100%Your percentages should add up to 100%

Same concept, but worded differentlySame concept, but worded differentlyAn 8.20g piece of magnesium combines completely with An 8.20g piece of magnesium combines completely with 5.40g of oxygen to form magnesium oxide. What is the 5.40g of oxygen to form magnesium oxide. What is the percent composition of the magnesium oxide?percent composition of the magnesium oxide?

Solution:Solution:

Find the mass of the compoundFind the mass of the compound

8.20g Mg + 5.40g O = 13.60 g8.20g Mg + 5.40g O = 13.60 g

Find the percent by mass of each elementFind the percent by mass of each element

%Mg = %Mg = 8.20g8.20g x 100% = x 100% = 60.29% of Mg60.29% of Mg

13.60 g 13.60 g

%O = %O = 5.40g5.40g x 100% = x 100% = 39.71 % of O39.71 % of O

13.60 g13.60 g

You try itYou try itWhat is the percent composition of glucose (CWhat is the percent composition of glucose (C66HH1212OO66))

figure out the molar mass:figure out the molar mass:

6 mol Carbon x 12.01g = 72.06 g of C6 mol Carbon x 12.01g = 72.06 g of C

12 mol Hydrogen x 1.01g = 12.12 g of H12 mol Hydrogen x 1.01g = 12.12 g of H

6 mol Oxygen x 16.00g = 6 mol Oxygen x 16.00g = 96.00 g of O96.00 g of O

molar mass = 180.18 g of glucosemolar mass = 180.18 g of glucose

Divide the answer of each atom by the molar mass and Divide the answer of each atom by the molar mass and multiply by 100multiply by 100

72.06 g of C / 180.18 g of glucose x 100% = 72.06 g of C / 180.18 g of glucose x 100% = 39.99 % C39.99 % C

12.12 g of H / 180.18 g of glucose x 100% = 12.12 g of H / 180.18 g of glucose x 100% = 6.73 % H6.73 % H

96.00 g of O / 180.18 g of glucose x 100% = 96.00 g of O / 180.18 g of glucose x 100% = 53.28 % O53.28 % O

Practice ProblemsPractice Problems

Calculate the percentage composition by mass of the Calculate the percentage composition by mass of the following compoundsfollowing compounds1) HgO1) HgO    

2) Na2) Na22SS

3) (NH3) (NH44))22SS    

4) Sr(NO4) Sr(NO33))22

1) 92.6% Hg,  7.37% O          1) 92.6% Hg,  7.37% O           2) 58.9% Na,  41.1% S          2) 58.9% Na,  41.1% S           3) 41.1% N,  11.8% H,   47.1% S          3) 41.1% N,  11.8% H,   47.1% S          4) 41.3% Sr,   13.2% N,  45.3% O4) 41.3% Sr,   13.2% N,  45.3% O