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Section 2: Finding Chemical Formulas

Calculate the total mass, percent composition, and empirical formula for different chemical compounds. Practice problems included.

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Section 2: Finding Chemical Formulas

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  1. Section 2:Finding Chemical Formulas

  2. Short Activity: Find the total weight of the objects and %mass. Total Mass:_____ Mass 1:____ % of total: Mass 2:____ % of total: Mass 3:____ % of total:

  3. Percent Composition: A measure of the amount of mass of elements in a compound by percent. Section 10.3 Example: Find the percent composition of the elements in H2O2. Step 1: Find the total molecular weight: MW = (1g/mol x 2) + (16 g/mol x 2) = 34 g/mol Step 2: Divide the mass of each element by the MW then multiply by 100. %H = (2 g/mol)/(34 g/mol) x 100 = 5.88% %O = (32 g/mol)/(34 g/mol)x100 = 94.11%

  4. 2) Fe2O3: MW = %Fe = %O = 3) C6H8O6: MW = %C = %H = %O =

  5. Extra Problems: Go to worksheet

  6. Finding Empirical Formulas: A material is separated into a pile of carbon and hydrogen. What could be a possible formula for this compound? There are two possible answers. To find empirical formulas you must know the number of ________ for each element.

  7. Chemical or Molecular Formula: Formula of elements in a compound.Empirical Formula: Gives the simplest ratio of elements in a compound. Section 10.3 cont. Find the molecular formula: Find the empirical formula: Hydrogen Peroxide:

  8. Empirical Formula Calculation: A compound contains 25.9% nitrogen and 74.1% oxygen. Find the empirical formula: Step 1: Find the number of moles of each. 25.9 g N 74.1 g O Step 2: Use the moles of each to write the empirical formula.

  9. Find the empirical formula for a sample that consist of 50.7% C, 4.2%H, and 45.1% O. C: H: O: If the actual molecular weight of the molecule is 142g/mol, find the molecular formula:

  10. Fat makes up a major portion of all soaps. A fat used in many soaps is 76.5% carbon, 12.2% hydrogen, and 11.3% oxygen. • What is its empirical formula? • What is the molecular formula of a fat with a molecular mass of 705 g/mol?

  11. Lab Practice Problem: A sample of a substance is determined to be composed of 0.89 grams of potassium, 1.18 grams of chromium, and 1.27 grams of oxygen. Calculate the empirical formula of this substance.

  12. Additional Problems: Pg 315 (65, 67, 68, 69)

  13. Quiz(20pts): Find the percent composition of the elements in Iron (III) Sulfate

  14. Quiz (20pts): Percent Composition Quiz Mg(OH)2 Fe2(SO4)3

  15. Quiz (20pts): Finding Percent Comps: Fe(OH)3 Mg(SO4)

  16. Quiz: (20pts) What is the empirical formula of a compound if a 51.9 g sample of it contains 9.1 g Na, 20.6 g Cr, and 22.2 g O?

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