1. Assignment *WE ARE ONLY ATTACHING THE STUFF IN WHITE TODAY* Page Number Empirical and Molecular Formulas Notes67-70 Calculating Percent Composition Example71 Percentage Composition worksheet (tape or staple top edge of worksheet and glue pocket underneath) 72 Empirical Formula Example73,74 Determining Empirical Formula worksheet (tape or staple top edge of worksheet and glue pocket underneath) 75 Molecular Formula Example76,77 Determining Molecular Formulas (True Formulas) worksheet. (tape or staple top edge of worksheet and glue pocket underneath) 78 Empirical and Molecular Formulas Practice Problems. (tape or staple top edge of worksheet and glue pocket underneath) 76 79

2. Empirical and Molecular Formulas

3. How Many Servings? How many servings of crackers do you think are in a box of cheez- its? Could you determine the number of servings if you were given the total mass of crackers (388g) and the mass of each serving (30g)?

4. Learning Objectives Define and calculate percent composition. Determine the empirical and molecular formulas for a compound from mass percent and actual mass.

5. Empirical and Molecular Formulas There are two different ways to represent chemical formulas: empirical formula molecular formula

6. Empirical and Molecular Formulas The empirical formula for a compound is the formula that shows the smallest whole-number ratio of the elements of a compound, and may or may not be the same as the actual molecular formula. The molecular formula for a compound specifies the actual number of atoms of each element in one molecule of a substance. Molecular formula

8. Percent Composition Before we can calculate the empirical or molecular formula, we need to know the percent composition of each element in the molecule.

9. Percent Composition

10. Percent Composition from Experimental Data

11. Percent Composition from the Chemical Formula

12. Calculating Percent Composition p71 Determine the percent composition of NaHCO 3. Step 1: Determine the molar mass of NaHCO3 and each element’s contribution.

13. Calculating Percent Composition p71 Step 2: Use the percent by mass equation.

14. continued on next slide

16. Homework Complete problems 1 & 2 (or 7 & 8)on page 79

17. Empirical Formula 1.Express percent by mass in grams. 2.Find the number of moles of each element. 3.Examine the mole ratio. 4.Write the empirical formula.

18. Empirical Formula from Percent Composition p73, 74 Determine the empirical formula for methyl acetate, which has the following chemical analysis: 48.64% carbon, 8.16% hydrogen, and 43.20% oxygen. Step 1: Assume that each percent by mass represents mass of the element in a 100.00-g sample. Step 2: Convert each mass to moles using a conversion factor—the inverse of the molar mass—that relates moles to grams.

19. Empirical Formula from Percent Composition p 73, 74 Step 3: calculate the simplest ratio of moles of elements by dividing the moles of each element by the smallest value in the calculated mole ratio.

20. Empirical Formula from Percent Composition p 73, 74 Step 4: multiply each number in the ratio by the smallest number—in this case 2—that yields a ratio of whole numbers.

21. Homework Complete problems 3 & 4 (or 9 & 10) on page 79

22. Molecular Formula To determine the molecular formula for a compound, the molar mass of the compound must be determined through experimentation and compared with the mass represented by the empirical formula. A molecular formula can be represented as the empirical formula multiplied by an integer n. Molecular formula = (empirical formula)n

23. Molecular Formula The molar mass of acetylene is 26.04 g/mol and the mass of the empirical formula (CH) is 13.02 g/mol. Because the molar mass of acetylene is two times the mass represented by the empirical formula, CH, the molecular formula of acetylene must contain twice the number of carbon and hydrogen atoms as represented by the empirical formula: C 2 H 2.

24. Molecular Formula 1.Express percent by mass in grams. 2.Find the number of moles of each element. 3.Examine the mole ratio. 4.Write the empirical formula. 5.Determine the integer that relates the empirical and molecular formula by dividing the molecular formula mass by the empirical formula mass. 6.Multiply the subscripst by n. 7.Write the molecular formula.

25. Determining a Molecular Formula p76, 77 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 g/mol. Determine the empirical and molecular formulas for succinic acid. Step 1: Assume that each percent by mass represents the mass of elements in a 100.00-g sample. Step 2: Step 2: Convert each mass to moles using a conversion factor—the inverse of the molar mass—that relates moles to grams.

26. Determining a Molecular Formula p76, 77 Step 3:calculate the simplest ratio of moles of elements by dividing the moles of each element by the smallest value in the calculated mole ratio.

27. Determining a Molecular Formula p76, 77 Step 4: multiply each number in the ratio by the smallest number—in this case 2—that yields a ratio of whole numbers.

28. Determining a Molecular Formula p76, 77 Step 5: Calculate the empirical formula mass using the molar mass of each element.

29. Determining a Molecular Formula p76, 77 Step 6: divide the experimentally determined molar mass of succinic acid by the mass of the emprirical formula to determine n.

30. Homework Complete problems 5 & 6 (or 11 &12) on page 79

31. Summary The percent by mass of an element in a compound gives the percentage of the compound’s total mass due to that element. The subscripts in an empirical formula give the smallest whole number ratio of moles of elements in the compound. The molecular formula gives the actual number of atoms of each element in a molecule or formula unit of a substance. The molecular formula is a whole-number multiple of the empirical formula.