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2011 topic 01 lecture 2 - empirical and molecular formulae

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David Young
David Young

The document discusses empirical and molecular formulas. It provides examples to show how to determine the empirical formula from percent composition data and how to then determine the molecular formula from the empirical formula and molar mass. Specifically, it shows how to calculate the empirical formula of adipic acid is C3H5O2 from its percent composition by mass. It then shows that if the molar mass of adipic acid is 146 g/mol, and the molar mass of C3H5O2 is 73.08 g/mol, then the molecular formula of adipic acid must be C6H10O4.

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IB Chemistry Power PointsTopic 1Quantitative Chemistrywww.pedagogics.caLecture 2Empirical and Molecular Formulae
Percent Composition by Mass
Recall: the relative molecular mass of H2SO4 can be determined as follows:Hydrogen 2 atoms × 1.008 = 2.01Sulfur 1 atom × 32.07 = 32.07Oxygen 4 atoms × 16.000 = 64.00 MR 98.09The percent composition by mass states the fraction of the total mass of each element in the compound. For example % sulfur
Similarly:Note the sum of these values should add to 100%
Empirical Formula DeterminationThat is “empirical” not Empire you fool!
DefinitionThe empirical formula is the formula that gives the simplest whole number ratio of atoms in a compound
An exampleCome to the dark sideAdipic acid contains 49.32% C, 43.84% O, and 6.85% H by mass. What is the empirical formula of adipic acid? 1. Assume 100g of the compound2. So, in 100 g of adipic acid, there would be 49.32 g of C, 43.84 g of O and 6.85 g of H.3. Determine moles of each element. For example: carbon
An example continuedI’m your father!Similarly:
Step 4. Divide each value of moles by the smallest of the values.
Step 5. If necessary, multiply each number by an integer to obtain all whole numbers.Carbon: 1.5Hydrogen: 2.5Oxygen: 1x 2x 2x 2352C3H5O2Empirical formula:
Alternatively, the empirical formula can be determined from mass composition data.Using the same example: a 21.6 g sample of adipicacid is decomposed yielding some carbon, 9.5 g of oxygen gas and 1.5 g of hydrogen gas. What is the empirical formula of adipic acid? First find the number of MOLES of each elementThen determine the lowest whole number ratio for the mole values
2011 topic 01 lecture 2 - empirical and molecular formulae
A MOLECULAR FORMULA is the actual chemical formula of a compound. The molecular formula will contain elements in the same ratio as the empirical formula. Molecular formulae are whole number multiples of empirical formulae.
We have determined that the empirical formula for adipic acid is C3H5O2. If the molar mass of adipic acid is 146 g/mol, what is the molecular formula of adipic acid?
1. Find the molar mass of C3H5O23(12.01 g) + 5(1.01) + 2(16.00) = 73.08 g2. Divide the molecular mass by the mass given by the empirical formula.146 ÷ 73.08 = 23. Multiply the empirical formula by this number to get the molecular formula.(C3H5O2) x 2 = C6H10O4

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2011 topic 01 lecture 2 - empirical and molecular formulae

  • 1. IB Chemistry Power PointsTopic 1Quantitative Chemistrywww.pedagogics.caLecture 2Empirical and Molecular Formulae
  • 3. Recall: the relative molecular mass of H2SO4 can be determined as follows:Hydrogen 2 atoms × 1.008 = 2.01Sulfur 1 atom × 32.07 = 32.07Oxygen 4 atoms × 16.000 = 64.00 MR 98.09The percent composition by mass states the fraction of the total mass of each element in the compound. For example % sulfur
  • 4. Similarly:Note the sum of these values should add to 100%
  • 5. Empirical Formula DeterminationThat is “empirical” not Empire you fool!
  • 6. DefinitionThe empirical formula is the formula that gives the simplest whole number ratio of atoms in a compound
  • 7. An exampleCome to the dark sideAdipic acid contains 49.32% C, 43.84% O, and 6.85% H by mass. What is the empirical formula of adipic acid? 1. Assume 100g of the compound2. So, in 100 g of adipic acid, there would be 49.32 g of C, 43.84 g of O and 6.85 g of H.3. Determine moles of each element. For example: carbon
  • 8. An example continuedI’m your father!Similarly:
  • 9. Step 4. Divide each value of moles by the smallest of the values.
  • 10. Step 5. If necessary, multiply each number by an integer to obtain all whole numbers.Carbon: 1.5Hydrogen: 2.5Oxygen: 1x 2x 2x 2352C3H5O2Empirical formula:
  • 11. Alternatively, the empirical formula can be determined from mass composition data.Using the same example: a 21.6 g sample of adipicacid is decomposed yielding some carbon, 9.5 g of oxygen gas and 1.5 g of hydrogen gas. What is the empirical formula of adipic acid? First find the number of MOLES of each elementThen determine the lowest whole number ratio for the mole values
  • 13. A MOLECULAR FORMULA is the actual chemical formula of a compound. The molecular formula will contain elements in the same ratio as the empirical formula. Molecular formulae are whole number multiples of empirical formulae.
  • 14. We have determined that the empirical formula for adipic acid is C3H5O2. If the molar mass of adipic acid is 146 g/mol, what is the molecular formula of adipic acid?
  • 15. 1. Find the molar mass of C3H5O23(12.01 g) + 5(1.01) + 2(16.00) = 73.08 g2. Divide the molecular mass by the mass given by the empirical formula.146 ÷ 73.08 = 23. Multiply the empirical formula by this number to get the molecular formula.(C3H5O2) x 2 = C6H10O4