1. Unit 1 Lecture 7: Percent Composition, Empirical and Molecular Formulas The student can justify the observation that the ratio of the masses of the constituent elements in any pure sample of that compound is always identical on the basis of the atomic molecular theory.

2. Mass Percent/ Percent Composition

3. NOTE: Subscripts indicate the mole ratio of elements present in a compound. For iron in iron(III) oxide, (Fe 2 O 3 ): Copyright © Cengage Learning. All rights reserved 3

4. Sodium hydrogen carbonate (NaHCO 3 ), also called baking soda, is an active ingredient in some antacids used for the relief of indigestion. Determine the percent composition each element in NaHCO 3. EXERCISE!

5. Consider separate 100.0 gram samples of each of the following: H 2 O, N 2 O, C 3 H 6 O 2, CO 2 – Rank them from highest to lowest percent oxygen by mass. H 2 O, CO 2, C 3 H 6 O 2, N 2 O Copyright © Cengage Learning. All rights reserved 5 EXERCISE!

6. Formulas A molecular formula of a compound, which shows the elements symbols and numerical subscripts, tells you the type and number of each atom in a molecule. The empirical formula is the formula that shows the smallest whole-number mole ratio of the compound’s elements. NOTE: Compound subscripts indicate the mole ratio of elements present in a particular compound.

7. Molecular formula (actual formula of the compound) – C 6 H 6 = (CH) 6 Empirical formula (shows simplest whole-number mole ratio) – CH Copyright © Cengage Learning. All rights reserved 7 Benzene Acetylene

9. Calculating Empirical Formula from Mass Percent You can calculate the empirical formula from percent by mass by 1.First, assuming you have 100.00 g of the compound. 2.Then, convert the mass of each element to moles. 3.Finally, divide each mole value by the smallest value. The empirical formula may or may not be the same as the molecular formula.

10. 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% carbon, 8.16% hydrogen, and 43.20% oxygen. Copyright © Cengage Learning. All rights reserved 10 EXERCISE!

12. Succinic acid is a substance produced by lichens. Chemical analysis indicates it 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. Copyright © Cengage Learning. All rights reserved 12 EXERCISE!

13. The composition of adipic acid is 49.3% C, 6.9% H, and 43.8% O (by mass). The molar mass of the compound is about 146 g/mol. – What is the empirical formula? C3H5O2C3H5O2 – What is the molecular formula? C 6 H 10 O 4 Copyright © Cengage Learning. All rights reserved 13 EXERCISE!

14. The mineral ilmenite is usually mined and processed for titanium, a strong, light, and flexible metal. A sample of ilmenite contains 5.41 g of iron, 4.64 g of titanium, and 4.65 g of oxygen. Determine the empirical formula for ilmenite. Copyright © Cengage Learning. All rights reserved 14 EXERCISE! Calculate the simplest ratio by dividing each mole value by the smallest value.

15. Hydrates Hydrates are compounds that incorporate water molecules in their crystalline structures. The ratio of moles of water to one mole of the compound is a small whole number. – Example: Hydrated copper(II) sulfate pentahydrate (CuSO 4 ·5H 2 O), has a compound:water ratio of 5:1.

16. CuSO 4 ·5H 2 O Anhydrous forms of hydrates are often used to absorb water, particularly during shipment of electronic and optical equipment. In chemistry labs, anhydrous forms of hydrates are used to remove moisture from the air and keep other substances dry.

17. Determining Hydrate Formula When heated, water molecules are released from a hydrate leaving an anhydrous compound. – Weigh hydrate, then heat to drive off the water. – Change in mass = mass of water lost – Convert the known masses of H 2 O and now anhydrous substance to moles. – Find ratio of moles of H 2 O to mole of anhydrous substance.

18. A mass of 2.50 g of blue, hydrated copper sulfate (CuSO 4 xH 2 O) is placed in a crucible and heated. After heating, 1.59 g of white anhydrous copper sulfate (CuSO 4 ) remains. What is the formula for the hydrate? Name the hydrate. Copyright © Cengage Learning. All rights reserved 18 EXERCISE! 1.Weigh hydrate, then heat to drive off the water. 2.Change in mass = mass of water lost 3.Convert the known masses of H 2 O and now anhydrous substance to moles. 4.Find ratio of moles of H 2 O to mole of anhydrous substance.

19. A 5.00 g sample of barium chloride hydrate was heated in a crucible. After the experiment, the mass of the solid weighed 4.26 g. Determine the number of moles of water that must be attached to BaCl 2. Copyright © Cengage Learning. All rights reserved 19 EXERCISE! 1.Weigh hydrate, then heat to drive off the water. 2.Change in mass = mass of water lost 3.Convert the known masses of H 2 O and now anhydrous substance to moles. 4.Find ratio of moles of H 2 O to mole of anhydrous substance.