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Percent Composition

Percent Composition. Percentage composition of a compound gives the relative amount of each element present. % = mass element x 100 mass compound The sum of percentages of each element in a compound should be = 100.

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Percent Composition

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  1. Percent Composition • Percentage composition of a compound gives the relative amount of each element present. • % = mass element x 100 mass compound • The sum of percentages of each element in a compound should be = 100

  2. Calculate the percent composition for carbon dioxide (CO2) • Calculate the molar mass for each element and the compound • C: 1 x 12.0 = 12.0 O: 2 x 16.0 = 32.0 + 44.0 g/mol • %C = 12 x 100 =27.3 % 44 % O = 32 x 100 = 72.7% 44 • Sum of percentages = 100 • Round percentages to the tenth.

  3. Calculate the percent composition for Al(OH)3 aluminum hydroxide • Calculate the molar mass for each element and the compound • Al: 1 x 27.0 = 27.0 H: 3 x 1.0 = 3.0 O: 3x 16.0 = 48.0 + 78.0 g/mol • %Al = 27 x 100 = 34.6% 78 % H = 3 x 100 = 3.8% 78 %O = 48 x 100 = 61.5% 78

  4. Percent Composition What is the percent carbon in C5H8NO4 (the glutamic acid used to make MSG monosodium glutamate), a compound used to flavor foods and tenderize meats? a) 8.22 %C b) 24.3 %C c) 41.1 %C

  5. Classworkp 344 #54-7

  6. Empirical and molecular formulas • If the identities of the elements of a compound are known and you also know its percentage composition, you can find the formula of the compound.

  7. Types of Formulas • Empirical Formula The formula of a compound that expresses the smallest whole number ratio of the atoms present. Ionic compounds formula are always empirical formula • Molecular Formula The formula that states the actual number of each kind of atom found in one molecule of the compound.

  8. Empirical Formula from % Composition A compound analyzed and found to contain 25.9% nitrogen and 74.1% O. What is the empirical formula of the compound? • Since percent means parts per 100, you can assume 100.g of the compound contains 25.9g N and 74.1 g O.

  9. Empirical Formula from % Composition A compound analyzed and found to contain 25.9% nitrogen and 74.1% O. What is the empirical formula of the compound? • Convert those values to moles: 25.9 g N 1 mol N = 1.85 mol N 14.0 g N 74.1 g O 1 mol O = 4.63 mol O 16.0 g O

  10. Empirical Formula from % Composition A compound analyzed and found to contain 25.9% nitrogen and 74.1% O. What is the empirical formula of the compound? • Divide each molar quantity by the smaller number of moles. 1.85 mol N = 1 1.85 4.63 mol O = 2.50 mol O 1.85

  11. Empirical Formula from % Composition A compound analyzed and found to contain 25.9% nitrogen and 74.1% O. What is the empirical formula of the compound? • The subscript for O is still NOT a whole number. Multiply both numbers by the smallest whole number that will convert both subscripts to whole numbers 1 mol N x 2 = 2 mol N 2.50 mol O x 2 = 5 mol O Empirical Formula: N2O5

  12. A sample of a brown gas, a major air pollutant, is found to contain 2.34 g N and 5.34g O. Determine the empirical formula for the compound. • convert grams to moles moles of N = 2.34g of N 1 mol = 0.167 moles of N 14.0 g moles of O = 5.34 g 1 mol = 0.334 moles of O 16.0 g • divide by smaller mole value: 0.167 mol N = 1 mol N 0.334 mol O = 2 mol O 0.167 0.167 Empirical formula : NO2

  13. CW p346 #58-61

  14. Molecular Formula • Some elements combine in more than one way. The molecular formula of a compound is either its empirical formula or a whole-number multiple of its empirical formula.

  15. Calculate the molecular formula of a compound whose molar mass is 60.0g/mol and empirical formula is CH4N. • Calculate molar mass of empirical formula: Molar mass CH4N: 30.0 g/mol • Divide molar mass molecular formula by molar mass empirical formula: 60.0 g/mol = 2 30.0 g/mol • Multiply the subscripts by this value. Molecular formula: C2H8N2

  16. The compound methyl butanoate smells like apples. Its percent composition is 58.8% C, 9.8%H, and 31.4% O. Its molar mass is 102 g/mol. What is its empirical formula? Its molecular formula? • Calculate empirical formula: • C5H10O2 • Calculate molar mass empirical formula: • 102 g/mol • Divide • 102/102= 1 • Multiply subscripts • C5H10O2

  17. Cw p 350 #62-66

  18. Hydrates • Hydrates are compounds that have a specific number of water molecules bound to them. • Naming hydrates: • Calcium chloride dihydrate: CaCl2 2H2O • Prefixes table 10.1 p351

  19. When a hydrate is heated, water evaporates leaving the anhydrous (without water) compound • Uses: • Dessicators: absorb moisture from air • Store solar energy

  20. Ex.1 A mass of 2.50g of blue, hydrated copper (II) sulfate (CuSO4 x H2O) is heated. After heating, 1.59g of anhydrous copper(II) sulfate (CuSO4) remains. What is the formula and the name for the hydrate?

  21. Ex. 2 Cerium (III) iodide (CeI3) occurs as a hydrate with the composition 76.3% CeI3 and 23.7% H2O. Calculate the formula for the hydrate. Cw. P361 #184-187

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