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Section 4: Empirical and Molecular Formulas

Section 4: Empirical and Molecular Formulas. Chemical Composition. Learning Goals. Explain what is meant by the percent composition of a compound. Determine the empirical and molecular formulas for a compound from mass percent and actual mass data. Percent Composition.

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Section 4: Empirical and Molecular Formulas

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  1. Section 4: Empirical and Molecular Formulas Chemical Composition

  2. Learning Goals • Explain what is meant by the percent composition of a compound. • Determine the empirical and molecular formulas for a compound from mass percent and actual mass data.

  3. Percent Composition • The percent by mass of any element in a compound can be found by dividing the mass of the element by the mass of the compound and multiplying by 100.

  4. Percent Composition • The percent by mass of each element in a compound is the percent composition of a compound. • Percent composition of a compound can also be determined from its chemical formula.

  5. Percent Composition

  6. Practice • What is the percent composition of phosphoric acid? • H3PO4

  7. Practice • H3PO4 • Determine the number of moles of each element in the compound (assume there is one mole of molecules).

  8. Practice • H = 3 mol, P = 1 mol, O = 4 mol • Determine the mass of each element by multiplying the number of moles of each element by their molar masses.

  9. Practice • H = 3.024 g, P = 30.97 g, O = 64.00 g • Find the total mass of one mole of the substance by adding together the masses of each element.

  10. Practice • 1 mol of H3PO4 = 97.99 g • Calculate the mass percent by dividing the mass of each element by the total mass of the substance, and then multiply by 100%.

  11. Practice • Mass percent of H = 3.086% • Mass percent of P = 31.61% • Mass percent of O = 65.31% 100.01%

  12. Practice • Sodium sulfate is used in the manufacturing of detergents. Identify the compound as ionic or covalent and then calculate the percent by mass of each element.

  13. Practice • Na2SO4

  14. Practice • Mass percent of Na = 32.37% • Mass percent of S = 22.58% • Mass percent of O = 45.05% 100.00%

  15. Empirical Formula • The empirical formula for a compound is the smallest whole-number mole ratio of the elements. • You can calculate the empirical formula from percent by mass by assuming you have 100.00 g of the compound. Then, convert the mass of each element to moles.

  16. 100% of SO3 is: 40.05% Sulfur 59.95% Oxygen 100 g of SO3 is: 40.05 g Sulfur 59.95 g Oxygen

  17. Empirical Formula • The empirical formula may or may not be the same as the molecular formula. • Molecular formula of hydrogen peroxide = H2O2 • Empirical formula of hydrogen peroxide = HO

  18. Example • Methyl acetate is a solvent commonly used in some paints, inks, and adhesives. Determine the empirical formula for methyl acetate, which has the following chemical analysis: 48.64% C, 8.16% H, and 43.20% oxygen.

  19. Example • 48.64% C, 8.16% H, and 43.20% oxygen • Assume that you have 100 g of the substance. You can now convert mass percent into grams.

  20. Example • 48.64 g C, 8.16 g H, and 43.20 g O • Convert these masses to moles of atoms by dividing by the molar masses of each atom.

  21. Example • 4.050 molC, 8.095 mol H, and 2.700 mol O • To find the whole number ratio of the atoms, divide all numbers by the smallest number.

  22. Example • 1.5 C : 3 H : 1 O • If needed, multiply the numbers by the smallest integer that will convert all of them to whole numbers.

  23. Example • C3H6O2

  24. Practice • The circle graph below gives the percent composition for a blue solid. What is the empirical formula for this solid?

  25. Practice • Determine the empirical formula for a compound that contains 35.98% Al and 64.02% S.

  26. Molecular Formula • The molecular formula specifies the actual number of atoms of each element in one molecule or formula unit of the substance. • Molecular formula is always a whole-number multiple of the empirical formula.

  27. Example • Succinic acid is composed of 40.68% carbon, 5.08% hydrogen, and 54.24% oxygen and has a molar mass of 118.1 g/mol. Determine the empirical and molecular formulas for succinic acid.

  28. Example • 40.68% C, 5.08% H, and 54.24% oxygen • molar mass of 118.1 g/mol • Determine the empirical formula for succinic acid.

  29. Example • C2H3O2 • Find the empirical formula mass by calculating the mass of 1 mol of the empirical formula.

  30. Example • Empirical formula mass = 59.04 g • To determine the molecular formula, first divide the molar mass by the empirical formula mass. Then multiply the whole number result by the empirical formula units.

  31. Example • (C2H3O2)2  C4H6O4 Molecular Formula

  32. Practice • A compound has 49.98 g of C and 10.47 g of H. The molar mass is 58.12 g/mol. Determine the molecular formula.

  33. Practice • A chemical has a molar mass of 110.0 g/mol and a percent composition shown in the circle graph. What is its molecular formula?

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