6.5 Mass & Number of Entities pp

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)

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)

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

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

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

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

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 × g/mol + 3 × g/mol = g/mol Step 2: Find the number of moles n = m ÷ M Al2O3 = 6.2 g ÷ g/mol = mol

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

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