Stoichiometry The Math of Chemical Reactions Unit 9

Stoichiometry  The study of the mathematical relationships between the substances in balanced chemical reactions Allows for calculation of quantities of reactants or products in a chemical reaction using the relationships found in the balanced chemical equation

Stoichiometry (cont.)  Allows for calculation of quantities of reactants or products in a chemical reaction  Example: According to the following reaction, how many grams of water can you make if you start with 10.0 grams of hydrogen gas? H 2 (g) + O 2 (g)  H 2 O (l)

Mole Ratios  Recall: Coefficients in a balanced chemical equation indicate the relative amounts of each substance involved in the reaction  In the following reaction: N 2 O 3 + H 2 O  2 HNO 3 One way to describe the ratios involved in the reaction would be “one molecule of dinitrogen trioxide plus one molecule of water yields two molecules of nitrous acid”

Mole Ratios  However, since these are only ratios, using the same equation, N 2 O 3 + H 2 O  2 HNO 3 It is equally true that, “one mole of dinitrogen trioxide plus one mole of water yields two moles of nitrous acid”  These mole relationships are called “ mole ratios ”

Mole Ratios  Example #1: What is the mole to mole ratio of oxygen to water in the following equation: 2 H 2 + O 2  2 H 2 O

Mole Ratios  Example #2: What is the mole ratio of sodium hydroxide to sodium sulfate in the following equation: H 2 SO NaOH  Na 2 SO H 2 O

Stoichiometric Calculations  Mole to Mole Calculations – converting from moles of one compound to moles of a different compound 1) Write and balance the equation if not done 2) The coefficients in front of the mentioned compounds serve as the conversion factor (mole ratio) between the two desired compounds 3) Use dimensional analysis & the mole ratio to convert from moles of compound #1 to moles of compound #2

Mole to Mole Calculations Ex #1: In the equation below, if 0.5 moles of magnesium hydroxide react, how many moles of water would be produced? 2 H 3 PO Mg(OH) 2  Mg 3 (PO 4 ) H 2 O

Mole to Mole Calculations Ex #2: In the equation below, if 0.5 moles of H 3 PO 4 react, how many moles of water would be produced? 2 H 3 PO Mg(OH) 2  Mg 3 (PO 4 ) H 2 O

 Mass to Mass Calculations 1) Convert from grams of given compound to moles of known compound (using molar mass) 2) Convert from moles of given to moles of unknown (using mole ratio) 3) Convert from moles of unknown to grams of unknown (using molar mass) Grams of Given Moles of Given Moles of Unknown Grams of Unknown Use molar mass of given Use molar mass of unknown Use mole ratio

Mass to Mass Calculations Master Formula for Mass to Mass Calculations

Mass to Mass Calculations Ex #1: In the balanced equation below, how many grams of carbon dioxide will be produced by the reaction of 108 grams of C 5 H 12 ? C 5 H O 2  5 CO H 2 O

Mass to Mass Calculations Ex #2: In the balanced equation below, how many grams of carbon dioxide will be needed to completely react with 11.5 grams of oxygen? C 5 H O 2  5 CO H 2 O

Percent Yield  Theoretical yield – amount of product that should be formed when the limiting reacting (the reactant that runs out first) is completely consumed  Actual yield – the actual amount of product produced in a reaction  Percent yield –

Percent Yield Example #1: If a student made 1.72 g of NaCl in the lab when they should have theoretically made 2.00 g, what is the percent yield?

Percent Yield Example #2: If a student made 4.5 g of H 2 SO 4 when they should have made 5.0 grams, what is the percent yield of the reaction? Does this percent yield make sense?