Law of Conservation of Matter Stoichiometry Mass-Mass Problems

What is Stoichiometry? The word stoichiometry derives from two Greek words: stoicheion (meaning "element") and metron (meaning "measure"). Stoichiometry deals with calculations about the masses and volumes of reactants and products involved in a chemical reaction. Every chemical reaction has it's characteristic proportions.  The method of obtaining these proportions from chemical formulas, equations, atomic weights and molecular weights, and the determination of what and how much is used and produced in chemical processes, is the major emphasis of Stoichiometry.

Proportional Relationships Stoichiometry mass relationships between substances in a chemical reaction based on the mole ratio Mole Ratio indicated by coefficients in a balanced equation 2 Mg + O2  2 MgO What is the mole ratio of magnesium metal to magnesium oxide? What is the mole ratio of magnesium oxide to oxygen? 2 : 2 2 : 1

Solving a Stoichiometry Problem Steps to follow to solve a stoichiometry mass-mass problem Write out and balance the chemical reaction Determine what is the given and unknown compounds Convert given mass to given moles by dividing the molar mass. Use the molar ratio from the balanced equation (unknown over known) to convert to unknown moles. Convert unknown moles to unknown mass by multiplying molar mass. Molar Ratio Molar Mass of unknown (g) 1 (mol) of known unknown mol Given Mass (g) Molar Mass of known (g) 1 (mol) of unknown known mol

Stoichiometry Problems When N2 combines with H2 they produce 5.63 g of NH3. How many grams of N2 were needed to produce the ammonia gas? N2 + 3 H2  2 NH3 4.63g 1.00g 5.63 g _____? 5.63 g NH3 1 mol NH3 1 mol N2 28.02 g N2 = 17.04 g NH3 2 mol NH3 1 mol N2 (5.63 x 1 x 1 x 28.02) ÷ (17.04 x 2 x 1) = 4.63 g N2 How many grams of H2 are needed for the reaction to take place? Remember the law of conservation of matter.

Stoichiometry Problems 6.50 grams of aluminum reacts with an excess of oxygen. How many grams of aluminum oxide are formed? 4 Al + 3 O2  2 Al2O3 6.50 g 5.78g 12.28g _____? 6.50 g Al 1 mol Al 2 mol Al2O3 101.96 g Al2O3 = 26.98 g Al 4 mol Al 1 mol Al2O3 (6.50 x 1 x 2 x 101.96) ÷ (26.98 x 4 x 1) = 12.28 g Al2O3 How much oxygen is needed for the reaction to take place?

Stoichiometry Problems Acetylene gas C2H2 burns in a combustion reaction for welding. How many grams of C2H2 are burned if the reaction produces 75.0 g of CO2? 2 C2H2 + 5 O2  4 CO2 + 2 H2O 22.19g _____? 75.0 g 75.0 g CO2 1 mol CO2 2 mol C2H2 26.04 g C2H2 = 44.01 g CO2 4 mol CO2 1 mol C2H2 (75.0 x 1 x 2 x 26.04) ÷ (44.01 x 4 x 1) = 22.19 g C2H2