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- 1. Percent Composition & Chemical Formulas
- 2. Percent Composition of a Compound • Definition: The percent by mass of an element in a compound • Ways to calculate % composition by mass: – 1) mass of element x 100% mass of compound – 2) mass of element in 1 mol of compound x 100% molar mass of compound
- 3. Example Problem #1: • Calculate the percent composition by mass of nitrogen in the compound NH3? – 1) mass of element x 100% mass of compound • 14g x 100 % = 82.4% N in NH3 17g
- 4. Example Problem #2: • Calculate the percent by mass of carbon in propane, C3H8? – 2) mass of element in 1 mol of compound x 100% molar mass of compound • Mass of C in 1 mol of C3H8 = 36g Molar mass of C3H8 44g x 100= 82% C
- 5. Empirical Formula • Gives the lowest whole-number ratio of the atoms or moles of the elements in a compound
- 6. Molecular Formula • Either the same as the empirical formula OR a simple whole-number multiple of the empirical formula • Chemists use mass spectrometer to determine molar mass
- 7. Example Problems: • Reduce the molecular formulas to the empirical formulas by dividing by the smallest whole number ratio • 3. P4O10 • 4. N2O4 • 5. C6H8O3 • 6. C100H220
- 8. Calculating Empirical Formula • The percent composition of a compound can be used to calculate the empirical formula of the compound • Steps: • 1) convert mass in grams to moles • 2)Divide each molar amount by the smaller # of moles • 3) Multiple each part of ratio by smallest whole # so subscripts are whole #s
- 9. Calculating Empirical Formula: • 7. Calculate the empirical formula of a compound that is 25.9% Nitrogen and 74.1% Oxygen.
- 10. Hints • On these stoichiometric problems, ask yourself: –What are you solving for? –What quantity is given?

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