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Molar Ratios From Empirical Formulas

Molar Ratios From Empirical Formulas. Empirical formula the smallest whole number ratio of atoms (or moles of atoms) of each element in a substance H 2 O 2 H atoms for every 1 O atom 2 moles of H atoms for every mole of O atoms. Molar Ratios From Empirical Formulas.

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Molar Ratios From Empirical Formulas

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  1. Molar Ratios From Empirical Formulas • Empirical formula • the smallest whole number ratio of atoms (or moles of atoms) of each element in a substance • H2O • 2 H atoms for every 1 O atom • 2 moles of H atoms for every mole of O atoms

  2. Molar Ratios From Empirical Formulas • The relative number of moles of each element in a substance can be used as a conversion factor called the molar ratio. • Molar ratio = moles element A mole of substance • Molar ratio = moles element A moles element B or

  3. Molar Ratios From Empirical Formulas • Fe2O3: • Molar Ratio = 2 moles of Fe mole Fe2O3 • Molar Ratio = 3 moles O mole Fe2O3 • Molar Ratio = 2 moles Fe 3 moles O

  4. Molar Ratios From Empirical Formulas • Molar ratios can be used to determine the number of moles of a particular element in a given substance. Moles A Molar Ratio Moles B

  5. Molar Ratios From Empirical Formulas Example: How many moles of Na+ ions are present in 2.5 moles of Na2SO4 ?

  6. Grams B Moles B Molar Ratios From Empirical Formulas • Remember, once you find the number of moles of a substance present, you can use: • Molar mass to find the number of grams • Avogadro’s number to find the number of atoms, ions, or molecules Moles A Molar Ratio N Molar mass Atoms B

  7. Molar Ratios From Empirical Formulas Example: What is the mass of iron present in 4.00 moles of Fe2O3?

  8. Percent Composition • The chemical formula for a compound can be used to determine the percent composition of the compound. • Percent composition: • the percentage of the mass contributed by each element in a substance • % Element X = (# atoms of X)(AW) x 100% FW of compound

  9. Percent Compositon Example:Calculate the % composition of C2H4O2 (i.e find %C, %H, and %O).

  10. Percent Composition Example:Calculate the % nitrogen in dinitrogen tetroxide.

  11. Calculating Empirical Formulas • The empirical formula for a compound can be determined experimentally using % composition data. • Four steps: • percent to mass • mass to mole • divide by smallest • multiple ‘til whole

  12. Calculating Empirical Formulas Example: Calculate the empirical formula for a substance that contains 34.63% C, 3.875% H and 61.50% O.

  13. Calculating Empirical Formulas

  14. Calculating Empirical Formulas Example: An iron compound contains 69.943 % Fe and 30.057 % O. Calculate its empirical formula.

  15. Calculating Empirical Formulas

  16. Using Empirical Formulas to Find Molecular Formulas • Empirical formula for a molecule is the smallest whole number ratio of atoms in the molecule. • The subscripts in the molecular formula must be some whole number multiple of the subscripts in the empirical formula: CH2O C2H4O2, C3H6O3, C4H8O4 X 2 X 3 X 4

  17. Using Empirical Formulas to Find Molecular Formulas • Steps: • Find the empirical formula • Calculate the formula weight for the empirical formula. • MW = the whole number ratio FW (empirical formula) between MW and FW Multiply the subscripts in the empirical formula by the whole number ratio.

  18. Using Empirical Formulas to Find Molecular Formulas Example: A certain substance with a molecular weight of about 54 amu has an empirical formula of C2H3. Determine its molecular formula.

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