Stoichiometry

Review of Molar Mass The molar mass (MM) of a compound is all the atomic masses for the molecule (or compound) added up. Ex. Molar mass of CaCl2 Avg. Atomic mass of Calcium = 40.08g Avg. Atomic mass of Chlorine = 35.45g Molar Mass of calcium chloride = 40.08 g/mol Ca + (2 X 35.45) g/mol Cl  110.98 g/mol CaCl2 20 Ca  40.08 17 Cl 35.45

Practice Ca3(PO4)2 Calculate the Molar Mass of calcium phosphate Formula = Masses elements: Ca = 40, P = 31, O = 16, Molar Mass = 3x40 + 2x31 + 8x16 = 310 g/mol Ca3(PO4)2

Everything must go through Moles!!! Calculations molar mass Avogadro’s number Grams Moles particles Everything must go through Moles!!!

Chocolate Chip Cookies!! 1 cup butter 1/2 cup white sugar 1 cup packed brown sugar 1 teaspoon vanilla extract 2 eggs 2 1/2 cups all-purpose flour 1 teaspoon baking soda 1 teaspoon salt 2 cups semisweet chocolate chips Makes 3 dozen 2 eggs 1 cup butter What is the ratio of eggs to butter to make 3 dozen cookies? What is the ratio of butter to chocolate chips? How many eggs would we need to make 9 dozen cookies? How much brown sugar would I need if I had 1 ½ cups white sugar? 1 to 2

Cookies and Chemistry…Huh!?!? Just like chocolate chip cookies have recipes, chemists have recipes as well Instead of calling them recipes, we call them chemical equations Furthermore, instead of using cups and teaspoons, we use moles Lastly, instead of eggs, butter, sugar, etc. we use chemical compounds as ingredients

4 moles of Na and 2 moles of Cl2 Chemistry Recipes Be sure you have a balanced reaction before you start! Example: 2 Na + Cl2  2 NaCl This reaction tells us that by mixing 2 moles of sodium with 1 mole of chlorine we will get 2 moles of sodium chloride How much Na and Cl2 would be need if we wanted to make 4 moles of NaCl? 4 moles of Na and 2 moles of Cl2

Practice 50 mol H2 = 50 mol H2O (they have a 1:1 ratio) Write the balanced reaction for hydrogen gas reacting with oxygen gas. 2 H2 + O2  2 H2O What if we had 50 moles of hydrogen, how much oxygen would react with it? How much water would be produced? 50 mol H2 x 1 mol O2 = 25 mol O2 2 mol H2 Mole ratio 50 mol H2 = 50 mol H2O (they have a 1:1 ratio)

Mole Ratios 5 moles Na 1 mol Cl2 2 mol Na = 2.5 moles Cl2 The mole ratio is used to switch between substances in a chemical equation. Example: How many moles of chlorine are needed to react with 5 moles of sodium? 2 Na + Cl2  2 NaCl 5 moles Na 1 mol Cl2 2 mol Na = 2.5 moles Cl2

Mole-Mole Conversions How many moles of sodium chloride will be produced if 2.6 moles of chlorine gas reacts with excess sodium metal? 2 Na + Cl2  2 NaCl 2.6 moles Cl2 x 2 mol NaCl = 5.20 mol of NaCl 1 mol Cl2

Mole-Mass Conversions Most of the time in chemistry, the amounts are given in grams instead of moles Example: How many grams of chlorine are required to react completely with 5.00 moles of sodium to produce sodium chloride? 2 Na + Cl2  2 NaCl Molar mass Mole ratio 5.00 moles Na 1 mol Cl2 70.90g Cl2 2 mol Na 1 mol Cl2 = 177g Cl2

Practice Given Quantity  convert to moles  mole ratio  convert to final units Calculate the mass in grams of Iodine required to react completely with 0.50 moles of aluminum. 2 Al + 3 I2  2 AlI3 0.50 moles Al X 3 moles I2 2 moles Al X 253.8 g I2 moles =190.35 g I2

Mass-Mole 10.0 g H2O 1 mol H2O 2 mol C2H6 18.0 g H2O 6 mol H20 We can also start with mass and convert to moles of product or another reactant Calculate the number of moles of ethane (C2H6) needed to produce 10.0 g of water 2 C2H6 + 7 O2  4 CO2 + 6 H20 Convert to moles Given quantity Multiply by the Mole ratio 10.0 g H2O 1 mol H2O 2 mol C2H6 18.0 g H2O 6 mol H20 = 0.185 mol C2H6

Consider : 4NH3 + 5O2  6H2O + 4NO How many grams of H2O are produced if 1.9 mol of NH3 are combined with excess oxygen? How many grams of O2 are required to produce 0.3 mol of H2O? # g H2O= 6 mol H2O 4 mol NH3 x 18.02 g H2O 1 mol H2O x 51.4 g H2O = 1.9 mol NH3 # g O2= 5 mol O2 6 mol H2O x 32 g O2 1 mol O2 x 8 g O2 = 0.3 mol H2O

Mass-Mass Conversions Most often we are given a starting mass and want to find out the mass of a product we will get (called theoretical yield) or how much of another reactant we need to completely react with it (no leftover ingredients!) Given Quantity  convert to moles  mole ratio  convert to final units

Mass-Mass Conversion 2.00g N2 1 mol N2 2 mol NH3 17.06g NH3 Ex. How many grams of ammonia are produced when 2.00g of nitrogen react with excess hydrogen. N2 + 3 H2  2 NH3 2.00g N2 1 mol N2 2 mol NH3 17.06g NH3 28.02g N2 1 mol N2 1 mol NH3 = 2.4 g NH3

Practice: Grams to Grams How many grams of calcium nitride are produced when 2.00 g of calcium reacts with an excess of nitrogen? 3 Ca + N2  Ca3N2 2.00 g Ca x 1 mole Ca x 1 mole Ca3N2 x 148 g Ca3N2 = 2.47 g Ca3N2 40 g 3 mol Ca 1 mole Given amount  change to moles  multiply by the mole ratio  change to (put the one on top that you grams are solving for)

Gases and Stoiochiometry Avogadro's Hypothesis - at the same temperature and pressure equal volumes of gas have the same number of particles. At STP 1 mole of any gas has a volume of 22.4 L STP = 0ºC and 1 atmosphere pressure

Gases and Stoichiometry 2 H2O2 (l) ---> 2 H2O (g) + O2 (g) Decompose 1.1 g of H2O2 in a flask with a volume of 2.50 L. What is the volume of O2 at STP? Bombardier beetle uses decomposition of hydrogen peroxide to defend itself.

Gases and Stoichiometry 2 H2O2 (l) ---> 2 H2O (g) + O2 (g) Decompose 1.1 g of H2O2 in a flask with a volume of 2.50 L. What is the volume of O2 at STP? Solution 1.1 g H2O2 1 mol H2O2 1 mol O2 22.4 L O2 34 g H2O2 2 mol H2O2 1 mol O2 = 0.36 L O2 at STP Molar volume

Gas Stoichiometry: Practice! A. What is the volume at STP of 4.00 g of CH4? 4.00 g CH4 x 1 mole x 22.4 L = 5.6 L of CH4 16 g 1 mole B. How many grams of CO2 are produced from the combustion of 20 grams CH4 at STP? CH4 + 2 O2  CO2 + 2 H2O 20 g CH4 x 1 mole x 1 mole CO2 x 22.4 L = 28 L CO2 16 g 1 mole CH4 1 mole