Percent Composition by Mass Use the Percent Composition by Mass Notes and the accompanying Worksheet as you view this power point presentation.
What is Percent Composition by Mass? a quantitative measure of what a compound or mixture is made of it tells us how much of each substance is in the compound or mixture it uses the mass of each substance and the total mass of the compound or mixture to arrive at a percentage for each substance let s calculate the percent composition by mass of a heterogeneous mixture known as a bacon and cheese omelet (the example in your notes)
Percent Composition by Mass for a Bacon & Cheese Omelet STEP 1 - Calculate the mass of each substance in the omelet: 71 g mass of eggs = 3 eggs = 210 g (2 sig figs) 1 egg In this calculation we needed to convert the number of eggs to the mass of eggs. To do this we used the equality 1 jumbo egg = 71 grams 71 g to create the conversion factor, 1 egg Now use this process to calculate the mass of bacon, cheese and milk in the omelet. You can check your answers online.
Bacon & Cheese Omelet (cont d) STEP 2 Calculate the total mass of the omelet: total massof omelet = 210 g + 51 g + 15 g + 62g = 340 g (sig figs to the tens digit) STEP 3 Calculate the percentage, by mass, of each substance: % eggs = mass of eggs 210 g 100% = mass of omelet 340g (2 sig figs) 100% = 61% Now use this process to calculate the percent composition for the other ingredients in the omelet. You can check your answers online.
Experimental vs. Theoretical Percent Composition by Mass Congratulations, you ve just calculated the percent composition by mass of a heterogeneous mixture! Let s move on to calculating percent composition by mass for compounds. We can calculate this percentage either experimentally by using lab data or theoretically by using the chemical formula. Experimental Method: The total mass of the sample is measured, the compound is decomposed into its individual elements, and the masses of the elements are measured. Theoretical Method: The chemical formula is used to determine the total mass of one molecule or formula unit, and the masses of its elements.
Percent Composition by Mass Experimental Method Follow the 4 step process outlined on page two of your notes to calculate the percent composition of hydrogen and oxygen in water using the experimental method. This is the example in your notes. 1. Total mass = 125 grams (given in the example problem) 2. The elements are hydrogen & oxygen (given in the problem) 3. Mass of hydrogen = 13.5 g and mass of oxygen = 111.5 g (both amounts are giving in the problem) 4. % hydrogen = 13.5g 125 g % oxygen = 111.5g 125 g 100% = 10.8% (3 sig figs) 100% = 89.2% (3 sig figs)
Percent Composition by Mass Theoretical Method Let s see how the theoretical method is used to solve this problem. The formula for water is H 2 O. 1. Total mass of 1 water molecule equals 1.0 gram gram 2 H atoms + 16.0 H atom O atom 1 O atom = 18.0 g 2. The elements are hydrogen & oxygen (given in the problem) 3. mass of hydrogen = 1.0 gram 2 H atoms = 2.0 g H atom mass of oxygen = 16.0 gram 1 O atoms = 16.0 g O atom 4. % hydrogen = 2.0g 100% = 11% (2 sig figs) 18.0 g % oxygen = 16.0g 18.0 g 100% = 89.2% (3 sig figs)
Percent Composition by Mass Worksheet Get your worksheet out and let s work #1 using the experimental method. #1. Total mass of sample = 1.26 g + 0.54 g = 1.80 g Elements in the compound are iron and oxygen. Mass of iron = 1.26 g and mass of oxygen = 0.54 g % iron = 1.26 g 1.80 g 100% = 70.0% (3 sig figs) % oxygen = 0.54 g 1.80 g 100% = 30. % (2 sig figs)
Percent Composition by Mass Worksheet Let s work #8 using the theoretical method. The formula is Na3PO4, so the total mass of 1 formula unit = 3(23.0 g) + 1(31.0 g) + 4(16.0 g) = 164.0 g Elements in the compound are sodium, phosphorous & oxygen. Mass of Na = 3(23.0 g) = 69.0 g, Mass of P = 1(31.0 g) = 31.0g Mass of O = 4(16.0 g) = 64.0 g % sodium = 69.0 g 164.0 g 100% = 42.1% (3 sig figs) % phosphorous = 31.0 g 164.0 g 100% = 18.9% (3 sig figs) % oxygen = 64.0 g 164.0 g 100% = 39.0% (3 sig figs)
Continue working on the problems in the worksheet. You can check your answers online. You must take the Percent Composition by Mass Quick Quiz #1 at the end of class. THIS IS AN INDIVIDUAL QUIZ FOR A GRADE, KEEP YOUR EYES ON YOUR COMPUTER SCREEN. You ll need about 5 to 6 minutes to finish it. The link to the quiz is on the Unit 7 web page under the Lesson 3 resources.