% Yield • Definitions • 1. Theoretical Yield • The maximum amount of product that can be formed from a given amount of reactants • In other words, the calculated amount predicted through stoichiometry
% Yield • Definitions • Actual Yield • The amount that is actually formed when the reaction is carried out in the laboratory.
% Yield = actual yield X 100 theoretical yield % yield will never be over 100% Most likely it will never even be 100%
Why will % yield never be 100% • Advantageous to add an excess of an inexpensive reagent to ensure that all of the more expensive reagents react • Reactant may not be 100% pure • Materials are lost during the reaction • If a reactions takes place in a solution it may be impossible to get all of the reactants or products out of the solution • If the reactions takes place at a high temperature, materials may be vaporized and escape into the air • Side reactions may occur • Example Mg burned in air. Some of Mg reacts with nitrogen reducing the amount of MgO produced. • Loss of product when filtering or transferring • If reactants are not carefully measured
% yield example problem • In a reaction between barium chloride and potassium sulfate, 3.89 g of barium sulfate is produced from 3.75 g of barium chloride. What is the percent yield?
% yield example problemanswer • BaCl2 + K2SO4 --> BaSO4 + 2 KCl 3. 75 g BaCl2 1 mol BaCl2 1 mol BaSO4 233.4 g BaSO4 208.3 g BaCl2 1 mol BaCl2 1 mol BaSO4 = 4.20 g BaSO4 3.89 g BaSO4 x 100 = 92.6 % 4.20 g BaSO4
% yield example problemyour turn 13.35 grams of magnesium hydroxide is produced when 42.50 grams of magnesium nitrate reacts with an excess of aluminum hydroxide. What is the percent yield?
% yield example problemanswer 3 Mg(NO3)2 + 2 Al(OH)3 --> 3 Mg(OH)2 + 2 Al(NO3)3 42.50g Mg(NO3)2 1mol Mg(NO3)2 3mol Mg(OH)2 58.3g Mg(OH)2 148.3g Mg(NO3)23mol Mg(NO3)2 1mol Mg(OH)2 = 16.71 g Mg(OH)2 % yield = 13.35 g Mg(OH)2 x 100 = 79.89 % Mg(OH)2 16.71 g Mg(OH)2