1. Copyright©2004 by Houghton Mifflin Company. All rights reserved.1 Introductory Chemistry: A Foundation FIFTH EDITION by Steven S. Zumdahl University of Illinois

2. Copyright©2004 by Houghton Mifflin Company. All rights reserved.2 8.3 Moles Apples can be purchased 3 ways: By count (3 for $1.00), by mass (1 lb = $0.89) or by volume (1 bushel = $10.00) Because atoms in chemistry are too small to “count” we need a very large number to help convert atoms to a practical form. We commonly use “words” to represent a number  dozen = 12, gross = 144, ream = 500 sheets A mole of anything equals 6.02 X 10 23 particles or Avogadro’s number

3. Copyright©2004 by Houghton Mifflin Company. All rights reserved.3 Representative Particle: the smallest unit of a substance that still retains the properties of that substance atoms in an element (except diatomic) formula units in an ionic compound molecules in a molecular compound 3 types of particles depending on the substance you are dealing with

4. Copyright©2004 by Houghton Mifflin Company. All rights reserved.4 3 types of particles… atoms (element) formula units (ionic) molecules (covalent) Molar Mass 6.02 X 10 23 22.4 L

5. Copyright©2004 by Houghton Mifflin Company. All rights reserved.5 Example 1- Calculate the number of moles in 10.0 g of Al ¬ Use the Periodic Table to determine the mass of 1 mole of Al 1 mole Al = 26.98 g = 27 g ­ Use this as a conversion factor for grams-to-moles

6. Copyright©2004 by Houghton Mifflin Company. All rights reserved.6 Use Avogadro’s Number to determine the number of atoms in 1 mole 1 mole Al = 6.02 x 10 23 atoms Use this as a conversion factor for moles-to- atoms Example 2- Calculate the number of atoms in 0.371 mol of Al

7. Copyright©2004 by Houghton Mifflin Company. All rights reserved.7 8.4 Mass of a Mole Gram Atomic Mass (GAM)- the number of grams of an element that is equal to the atomic mass in amu. For compounds: The formula tells us the kind and number of atoms SO 3 = 1 Sulfur and 3 oxygen Mass= 1(32 g) + 3 (16 g) = 80 g Gram Molecular Mass (covalent)- the mass of 1 mole of any molecular compound Gram Formula Mass (ionic)- the mass of 1 mole of any ionic compound

8. Copyright©2004 by Houghton Mifflin Company. All rights reserved.8 Molar Mass The molar mass is the mass in grams of one mole of a compound (generic term) The relative weights of molecules can be calculated from atomic masses water = H 2 O = 2 (1g) + 16 g = 18 g

9. Copyright©2004 by Houghton Mifflin Company. All rights reserved.9 Example 3- Calculate the molar mass of sodium carbonate (Na 2 CO 3 ) So, the molar mass of sodium carbonate, would be Na: 2 (23 g) C: 1 (12 g) + O: 3 (16 g) 106 g We can use Molar mass to convert from grams of any substance to moles

10. Copyright©2004 by Houghton Mifflin Company. All rights reserved.10 Carbon tetrachloride (CCl 4 ) is a covalent compound so particles will be molecules Example 4- Calculate the number of molecules in 15.0 g of carbon tetrachloride

11. Copyright©2004 by Houghton Mifflin Company. All rights reserved.11 8.5 Percent Composition Percentage of each element in a compound by mass Can be determined from the formula of the compound or the experimental mass analysis of the compound The percentages may not always total to 100% due to rounding

12. Copyright©2004 by Houghton Mifflin Company. All rights reserved.12 Example 5- Determine the Percent Composition from the Formula C 2 H 5 OH Determine the molar mass of the compound 1 mole C 2 H 5 OH = 46 g

13. Copyright©2004 by Houghton Mifflin Company. All rights reserved.13 Example 6- An 8.20 g piece of magnesium combines completely with 5.40 g of oxygen gas to form a compound, What is the % composition of the compound? % Mg = mass of element 8.20 g X 100 = 60.3% mass of compound 13.6 g % O = mass of element 5.40 g X 100 = 39.7% mass of compound 13.6 g

14. Copyright©2004 by Houghton Mifflin Company. All rights reserved.14 8.6 Empirical Formulas The simplest, whole-number ratio of atoms in a molecule is called the Empirical Formula  can be determined from percent composition or combining masses The Molecular Formula is a multiple of the Empirical Formula

15. Copyright©2004 by Houghton Mifflin Company. All rights reserved.15 Find the greatest common factor (GCF) of the subscripts Divide each subscript by the GCF to get the empirical formula C 20 H 12 = (C 5 H 3 ) 4 Empirical Formula = C 5 H 3 Determine the Empirical Formula of Benzopyrene, C 20 H 12 Example 7-

16. Copyright©2004 by Houghton Mifflin Company. All rights reserved.16 Calculating Empirical Formulas 3 Steps to calculate 1- % to grams 2- grams to moles 3- divide by smallest moles

17. Copyright©2004 by Houghton Mifflin Company. All rights reserved.17 Convert the percentages to grams by assuming you have 100 g of the compound Example 8- Determine the Empirical Formula of Acetic Anhydride if its Percent Composition is 47% C, 6.0% H and 47% O Step 1- % to grams

18. Copyright©2004 by Houghton Mifflin Company. All rights reserved.18 Step 2- grams to moles

19. Copyright©2004 by Houghton Mifflin Company. All rights reserved.19 Step 3 Divide each by the smallest number of moles…and hope for whole numbers. If any of the ratios is not a whole number, multiply all the ratios by a factor to make it a whole number If ratio is ?.5 then multiply by 2; if ?.33 or ?.67 then multiply by 3; if ?.25 or ?.75 then multiply by 4 Multiply all the Ratios by 3 Because C is 1.3 X 3 = 4 X 3 = 3 X 3 = 6 C4H6O3C4H6O3

20. Copyright©2004 by Houghton Mifflin Company. All rights reserved.20 8.8 Molecular Formulas The molecular formula is a multiple of the empirical formula #(EF) = MF To determine the molecular formula you need to know the empirical formula and the molar mass of the compound

21. Copyright©2004 by Houghton Mifflin Company. All rights reserved.21 Example 9-Determine the Molecular Formula of Benzopyrene if it has a molar mass of 252 g and an empirical formula of C 5 H 3 Determine the empirical formula (May need to calculate it as previous example) Determine the molar mass of the empirical formula C 5 H 3 = 63 g n = molar mass (from problem) empirical formula mass Multiply the empirical formula by the calculated factor to give the molecular formula 4 (C 5 H 3 ) = C 20 H 12