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Stoichiometry and Compound Formulas: Element Composition, Percent Composition, Empirical Formulas, Molecular Formulas, a

This section covers the element composition of compounds, determining percent composition and empirical formulas from formulas or percent compositions, finding molecular formulas from empirical formulas, and determining the number of waters of hydration in hydrated compounds.

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Stoichiometry and Compound Formulas: Element Composition, Percent Composition, Empirical Formulas, Molecular Formulas, a

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  1. Section 3.2 Stoichiometry and Compound Formulas

  2. In this section… • Element composition • Percent composition from formulas • Determining empirical formulas • Determining molecular formulas • Hydrated compounds Compound Stoichiometry

  3. You have a sample of a compound with known mass. What are the amounts of each element present? Element Composition:

  4. You have 2.50-mol sample of CH3CO2H. What are the amounts of each element present? Element Composition:

  5. A compound is formed from two or more elements in a set ratio of atoms. The ratio of atoms gives the chemical formula. The ratio of masses of those atoms gives the percent composition. Percent Composition:

  6. Determining Percent Composition:

  7. formula %composition *** If given just the formula, assume you have 1 mol of the compound What is the percent composition of each element in N2O5? Determining Percent Composition:

  8. %composition formula *** If given just the percent compositions, assume you have 100 g of the compound Determining Empirical Formulas from Percent Composition: • Process: • for each element, mass present = % comp in g • convert g to mol for each element • compare mol of each element to get formula

  9. Use these % composition values to determine the empirical formula: Determining Empirical Formulas from Percent Composition:

  10. Some useful fraction-to-formula ratios: 1.2 1 + 1/5 6/5 A5B6 1.25 1 + ¼ 5/4 A4B5 1.33 1 + 1/3 4/3 A3B4 1.4 1 + 2/5 7/5 A5B7 1.5 1 + ½ 3/2 A2B3 1.6 1 + 3/5 8/5 A5B8 1.67 1 + 2/3 5/3 A3B5 1.75 1 + ¾ 7/4 A4B7 1.8 1 + 4/5 9/5 A5B9 Determining Empirical Formulas from Percent Composition:

  11. Empirical vs. Molecular Formulas Empirical Formula: Molecular Formula: If you know the empirical formula, how do you find the molecular formula? answer: molar masses of the two Example: Molar mass of empirical formula (CH2) = 14.03 g/mol Molar mass of compound measured by experiment to be 56.11. Molecular Formula =

  12. %composition empirical formula molecular formula molar mass Determining Molecular Formulas from Percent Composition and Molar mass • Process: • for each element, mass present = % comp in g • convert g to mol for each element • compare mol of each element to • get empirical formula • compare empirical formula molar mass to • compound molar mass

  13. Determining Molecular Formula A compound has molar mass of 110.11 g/mol and contains: 65.45% C 5.493% H 29.06% O Empirical Formula = Empirical Formula (C3H3O)molar mass = 55.06 g/mol Molecular formula =

  14. Hydrated Compounds

  15. Hydrated Compounds How do you determine the number of waters of hydration? Answer: heat off the water and see how much mass decreases mass loss = mass H2O moles H2O = # of waters of final mass = mass anhydrous compound moles hydration anhydrous compound

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