1 Percent Composition Honors Chemistry

2 n n The relative amounts of each element in a compound are expressed as the percent composition, which is the percent by mass of each element in the compound. n n It is sometimes useful to know the relative amounts of each of the components compound. n n In other words, what is the percentage of nitrogen that you get from the compound sodium nitrate (NaNO 3 ) ?

3 n n The amount of fertilizer that you put on the grass in your yard or on plants in your garden is important. n n The relative amount, or the percentage, or each nutrient in the fertilizer is also important. n n In spring you may use a fertilizer that has a relatively high percentage of nitrogen, which “greens” you plants. n n In the fall you may use a fertilizer that has a higher percentage of potassium to strengthen the root system of your plants. Real Life Example

4 Calculating Percent Composition of a Compound n The following general formula is used to determine the percent composition of each element in a compound:   Mass of element (in 1 mole of compound) X 100   Molar Mass of Compound

5 What is the percent composition of each element in Ca(NO 3 ) 2 ? n Step 1: Find the molar mass of Ca(NO 3 ) 2 n Ca 40.1 grams/mole = 40.1 g/mol n N 2 x (14.0) grams/mole = 28.0g/mol n O 6 x (16.0) grams/mole = 96.0 g/mol n g/mole

6 Step 2: Divide the mass of each element by the molar mass and multiply by 100 to get the percent. n % Ca = 40.1g x 100% = 24.4 % Calcium n 164.1g n n % N = 28.0g x 100% = 17.1% Nitrogen n g n n % O = 96.0g x 100% = 58.5 % Oxygen n 164.1g

7 *Note percent composition can also be done given masses of a compound and each element in the compound.   Calculate the percent composition of a compound that is made of 29.0 grams of Ag with 4.30 grams of S g Ag 33.3 g total X 100 = 87.1 % Ag 4.30 g S 33.3 g total X 100 = 12.9 % S Total = 100 %

8 To determine mass of an element that could be recovered from a compound n n How many grams of silver can be recovered from a 500gram sample of AgNO 3 ? % Ag = g Ag x g AgNO 3 = 63.52% Ag What is 63.52% of 500grams? (0.6352) (500) = grams Ag

9 Using the Mole Concept n n You can also calculate the last example as follows. 500 g AgNO 3 1 mole AgNO g AgNO 3 1 mole Ag 1 mol AgNO g Ag 1 mol Ag = grams Ag 500 g AgNO 3