1. 6.5 Mass & Number of Entities pp. 278 - 282

2. Avogadro's constant (N A ) is the link between the mass of a substance and the number of entities present. Yesterday we learned that you can figure out the number of moles (n) if you know mass (m)

3. With a further equation you can convert number of moles (n) to number of entities (N): N = n × N A N – number of entities n – number of moles (mol) N A – avagadro’s constant (# of entities / mol)

4. Example # 1 How many atoms are there in a 2.00 kg bar of silver Step 1: Find molar mass Step 2: Find the number of moles Step 3: Find the number of atoms

5. Example # 1 Step 1: Find the molar mass M Ag = 107.87 g / mol Step 2: Find the number of moles n = m ÷ M Al n = 2000g ÷ 107.87 g / mol = 18.54 mol Step 3: Find the number of atoms N = n × N A N = 18.5408 mol × 6.023 ×10 23 atoms / mol N = 1.12 × 10 25 atoms

6. Example # 2 How many molecules are in a 12.0 mg sample of CO 2 ? Step 1: Find molar mass Step 2: Find the number of moles Step 3: Find the number of molecules

7. Example # 2 Step 1: Find the molar mass M CO2 = M C + 2M O = 12.01 g/mol + 2 ×16.00 g/mol = 44.01 g/mol Step 2: Find the number of moles n = m ÷ M CO2 n = 0.0120g ÷ 44.01 g/mol = 0.52812 mol Step 3: Find the number of molecules N = n × N A N = 0.52812 mol × 6.023 ×10 23 molecules / mol N = 3.18 × 10 23 molecules

8. Example # 3 How many Aluminum ions are present in 6.2g of Al 2 O 3 ? Step 1: Find molar mass M Al2O3 = 2M Al + 3M O = 2 × 26.98 g/mol + 3 × 16.00 g/mol = 101.96 g/mol Step 2: Find the number of moles n = m ÷ M Al2O3 = 6.2 g ÷ 101.96 g/mol = 0.06080816 mol

9. Example # 3 Step 3: Find number of molecules N = n × N A = 0.06080816 mol × 6.023 ×10 23 molecules / mol = 3.6624 × 10 22 molecules Step 4: Find the number of Aluminum atoms N Al atoms = N molecules × 2 atoms / molecule = 3.6624 × 10 22 molecules × 2 atoms / molecule = 7.32 × 10 22 atoms

10. Homework Read pp. 278 – 282 Answer # 1 – 8 on p. 283