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Percent Composition, Empirical and Molecular Formulas

Percent Composition, Empirical and Molecular Formulas. Percent Composition. Percentage by mass of each element in a compound Example: Determine the % of hydrogen and oxygen in water 1. Start with formula: H2O 2. Calculate formula mass: 18.015g/mol

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Percent Composition, Empirical and Molecular Formulas

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  1. Percent Composition, Empirical and Molecular Formulas

  2. Percent Composition • Percentage by mass of each element in a compound • Example: Determine the % of hydrogen and oxygen in water 1. Start with formula: H2O 2. Calculate formula mass: 18.015g/mol 3. Determine % of total mass for each element H: 2.016/18.015 = .11 × 100% = 11% hydrogen O: 15.999/18.015 = .89 × 100% = 89% oxygen

  3. Practice Problem #1 • Determine the % composition of Sulfur in H2SO4

  4. Practice Problem #2 • How much Na can be recovered from 45g of NaCl?

  5. Empirical vs. Molecular Formulas • Empirical Formula: shows the ratio of elements in a compound • Molecular Formula: shows the actual number of atoms in a compound • Example: glucose empirical: CH2O molecular: C6H12O6

  6. Calculating Empirical Formulas • Assume you have 100g of a sample • Use % composition and convert to grams • Divide number of grams of element by its molar mass to determine # of moles • Divide all of the mole numbers by the smallest mole number to determine ratios • Write empirical formula

  7. Example #1 • Determine the empirical formula for a compound that is 80% carbon and 20% hydrogen.

  8. Example #2 • Determine the empirical formula for a compound that is 38.7% Mg and 61.3% F.

  9. Finding Molecular Formulas • Example #1: A white powder was analyzed and found to have an empirical formula of P2O5. The compound has a molar mass of 283.88g. What is the molecular formula? • Empirical Formula Mass P2O5 = 141.943g • Divide actual molar mass by empirical mass 283.88g/141.943g = 1.99 (can round up to 2) • (P2O5)2 → P4O10

  10. Finding Molecular Formulas • Vitamin C has been analyzed and found to contain 40.9% carbon, 4.58% hydrogen, and 54.5% oxygen. The molar mass of vitamin C is 176g. What is the molecular formula? (HINT: find empirical formula first!)

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