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Empirical Formulas

Empirical Formulas. Unit 5. Percent Composition. Percent composition is a number that tells us how much of an element makes up a compound The formula used to calculate percent composition is:.

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Empirical Formulas

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  1. Empirical Formulas Unit 5

  2. Percent Composition • Percent composition is a number that tells us how much of an element makes up a compound • The formula used to calculate percent composition is:

  3. An 18.02 g sample of H2O contains 2.02 g of H and 16.00 g of O. What is the percent by mass for each element? 11.2 % H 88.80 % O Sample Problem

  4. Calculating Percent Composition • Step #1: Calculate the molar mass of the compound • Step #2: Use the atomic mass of each element and the formula for percent composition to calculate the percent composition of each element • Example: NaHCO3

  5. Empirical Formula • If we know the percent composition for each element of a compound, we can calculate the compound's empirical formula • An empirical formula is the formula with the smallest whole number ratio of the elements

  6. Calculating Empirical Formula • Step #1: Convert grams of each element to moles • Step #2: Divide each of these numbers by the smallest number • Example: • What is the empirical formula for a compound that contains 6.93 g of oxygen and 0.43 g of hydrogen?

  7. A sample of unknown origin contains 48.64% C, 8.16% H, and 43.20 % oxygen. What is the empirical formula for this compound? C3H6O2 Sample Problem

  8. Molecular Formula • A molecular formula tells you exactly the number of atoms present in a molecule of a compound • Molecular formulas can be calculated by using empirical formulas

  9. Calculating Molecular Formulas • Molecular formulas can be calculated using the equation below:

  10. The empirical formula for succinic acid is C2H3O2. The molar mass of succinic acid is 118.1 g/mol. What is the molecular formula for succinic acid? C4H6O4 Sample Problem

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