Chapter 9 STOICHIOMETRY

What’s it mean? Greek stoikheion, meaning elementelement metron, meaning measuremeasure In English….chemical recipe

IDEAL STOICHIOMETRY Ideal -

STEPS to SOLVING PROBLEMS

Go with the flow…. chart to solve the 4 simple types of problems

1. MOLE – MOLE What are we calculating?

EX. How many moles of ammonia are produced when 6 moles of hydrogen gas react with an excess of nitrogen gas? Balanced reaction:

Ex. How many moles of nitrogen are needed for the reaction to go to completion?

3. MASS – MOLE What are we calculating? Moles of any product or reactant, given grams of any product or reactant. Ex. How many moles of mercury (II) oxide are needed to produce 125 grams of oxygen in a decomposition reaction?

2. MOLE - MASS What are we calculating? Grams of any product or reactant, given moles of any product or reactant. Ex. When magnesium burns in air, it combines with oxygen to form magnesium oxide. What mass of magnesium oxide is produced from 2.00 moles of magnesium?

Ex. How many moles of mercury are produced by the reaction?

4. MASS - MASS What are we calculating? Ex. Nitrous oxide is sometimes used as an anesthetic in dental work. It is produced when ammonium nitrate is decomposed into dinitrogen monoxide and water. How many grams of ammonium nitrate are needed to produce 33.0 g of N 2 O?

How many grams of water are produced during the reaction?

MORE EXAMPLES…. Acetylene gas (C2H2) is used in welding and produces a very hot flame when burned in pure oxygen. How many grams of each product are produced when 25 kg of acetylene burns completely?

PICK 1 1. How many grams of silver nitrate are needed to react with 12.0 grams of sodium chloride? 2. How many grams of silver chloride could be produced during the reaction if 3.50 grams of sodium chloride are used? 3. How many moles of sodium nitrate will form in solution if 5.00 grams of silver chloride precipitated? 4. Given 15.0 ml of a solution of sodium chloride (1.03 g/ml = density), determine how many moles of silver chloride could precipitate during the reaction. 5. If 20.0 grams of silver chloride are produced, how many moles of sodium chloride and silver nitrate must have been mixed?

MOLAR VOLUME of a GAS Rather than weighing gaseous products and reactants, it is often easier to consider the VOLUME of gas reacted or produced. For this to be useful in a stoichiometry problem, we must relate the volume of a gas to a number of moles. MOLAR VOLUME OF A GAS STP =

Ex. Using molar volume Acetylene burns in oxygen. If L of acetylene is burned at STP, what volume of carbon dioxide is produced? How many moles of water will be produced? If 84 L of acetylene are used, how many liters of oxygen are needed for complete combustion?

Pick 1 A single displacement reaction between magnesium and hydrochloric acid produced hydrogen gas at STP. 1. If 5.0 grams of magnesium ribbon are used, how many L of hydrogen are produced? 2. If you want to produce 250 ml of hydrogen, how many grams of magnesium must be used? 3. If HCl has a density of 1.02 g/ ml. How many ml of HCl are needed to produce 2.0 L of hydrogen gas? 4. If 0.50 grams of magnesium react, how many grams of MgCl 2 will form? 5. If 50.0 grams of HCl react with an excess of Mg, how many L of H 2 will form?

PERCENT YIELD and STOICHIOMETRY THEORETICAL YIELD ACTUAL YIELD

% YIELD % yield =

% yield examples A reaction is supposed to produce 200. grams of HCl. If it actually yield 178 grams of HCl, how efficient is the reaction?

If chlorine and hydrogen are combined to form HCl, if you are to obtain a 98% yield and you start with 3.0 moles of hydrogen, what amount of HCl is produced? What is the % yield of HCl if you start with 5.0 moles of hydrogen and obtain 345 grams of HCl?

LIMITING REAGENT LIMITING REACTANT We have already seen that reaction conditions are not ideal limit the amount of product produced when dealing with % yield calculations. Now suppose one reactant is completely consumed during the reaction? What is a limiting reactant? Excess reactant?

Steps for determining the LR and EXCESS reagent 1. Set up stoichiometry problems beginning with the given amount of each reactant and determine the amount of ____________that may be formed. 2. Select the reagent that limits the amount of product by choosing the one that__________________. This is your limiting reagent. 3. Determine how much of the other reactant is used starting with _______________________________________. 4. __________the amount of excess reactant used from the given amount to find the amount left over.

LIMITING REAGENT example Zinc and sulfur react to form zinc sulfide. If grams react with grams of sulfur determine the following: a. What is the limiting reagent?

b. Which reactant is in excess? c. How much zinc sulfide could be produced? What is the maximum amount of product?

d. How many grams of excess reagent remain?