Percent Composition LG: I can calculate percent composition given a chemical formula or experimental data

Percent Composition When unknown substances undergo chemical analysis, the results are reported as percent composition, which is the percentage of the total mass made up by each element – The fertilizer below contains 18% nitrogen, 24% phosphorous, and 6% potassium Percent composition can be determined by experimental data or from a chemical formula

Law of Definite Proportions Percent composition is useful in identifying unknown substances because all chemical compounds abide by the law of definite proportions: “The elements in a given compound are always present in the same proportion by mass” – Example: WaterHydrogen Peroxide FormulaH2OH2OH2O2H2O2 Ratio of atoms2:11:1 Mass ratio2:16 or 1:82: 32 or 1:16

Example 1: Use the following data to determine percent composition – A mg Aspirin tablet, C 9 H 8 O 4, contains mg of carbon and 8.08 mg of hydrogen 1)Divide the mass of each element by the total mass of the sample and multiply by 100% 1)Subtract known percentages from 100 to determine percent composition of oxygen 1)Include a statement to summarize results

Example 2: Calculate the percent composition of each element in Mg(OH) 2 1)Find molar mass of compound 1)Divide mass of each element by total mass and multiply by 100% 1)Check that percentages add up to 100% 1)Include a statement to summarize results

Practice 1) Determine the percent composition for potassium sulfate 2 ) Determine the percent composition of a 2.50 g sample is found to contain g of calcium and 1.60 g of chlorine

Mixtures and Alloys Percent composition can also be used to analyze: – Alloys: different metals melted and blended together – Mixtures: Food chemists are responsible for verifying nutrition information on labels (ie. Salt content)

Homework Old Book – Pg. 179 # 1-3 Pg. 184 # 5-8 New Book – Pg. 288 # 1, 2, 5, 6, 7 Review lab procedure for tomorrow and complete pre-lab calculation