Stoichiometry

The Basics  What is stoichiometry?  A method of determining the quantities of products produced in a chemical reaction or what amount of a reactant needed to carry out a chemical reaction.  What do the stoichiometric coefficients tell us about the chemical reaction and the ratio between the chemical compounds involved?

MOLE RATIO ! ! !

Example 1: Milk of magnesia, Mg(OH) 2, reacts with stomach acid in a neutralization reaction. What is the mass (in grams) of magnesium chloride produced when 3.00g of milk of magnesia is taken?

More Practice with the Basics 1) _____N 2 H 4 + ____O 2  ____N 2 + _____H 2 O (N 2 H 4 is rocket fuel) We have 5.20 mol of N 2 H 4. How many moles of O 2 do we use up in the reaction? 2) ____C 6 H 12 O 6 + ____O 2  ____CO 2 + _____H 2 O If you eat 10.0 grams of sugar, how many grams of oxygen do you need to breathe to carry out this reaction and get energy? 

Example 2: True Test of Stoichiometry Skills Gaseous ammonia is placed into an aqueous solution that is 65% H 2 SO 4 by mass with a density of 1.55 g/mol. Ammonium sulfate is produced from this reaction and used as fertilizer. How many milliliters of sulfuric acid are needed along with 1.00 kg NH 3 to produce ammonium phosphate?

Example 3:  Liquid bromine is reacted with aluminum to produce aluminum bromide.  2Al (s) + 3Br 2 (l) 2AlBr 3 (s)  If 450 ml of bromine reacts with excess aluminum, how much aluminum bromide will be made? The density of bromine is 3.1 g/ml

Limiting Reactant  Chemical compound that “limits” or determines how much product can be produced  All of the compound is consumed in the reaction  Excess reactant— chemical compound not consumed in the reaction, plenty of the compound is available for the chemical reaction

Steps to find the limiting reactant  Two stoichiometry problems in ONE ! 1) Write a balanced equation 2) Find the amount of product (in grams) produced from each reactant amount. 3) The reactant producing the SMALLEST product is the limiting reactant.

Example 4: Aspirin synthesis: 2C 7 H 6 O 3 + C 4 H 6 O 3 2C 9 H 8 O 4 + H 2 O When 20.0g of C 7 H 6 O 3 and 20.0g of C 4 H 6 O 3 react, which is the limiting reactant? What is the mass (in grams) of aspirin produced?

Percent Yield  “ yield —” the amount of product actually made through a chemical reaction. Why is this value important?  Theoretical yield— calculated amount of product  Actual/Experimental yield– the “true/actual” amount of product made in the chemical reaction, usually less than theoretical  How is this value different than percent error?

Percent Yield  = Actual yield x 100% Theoretical yield Ex % of Mg 3 N 2 was produced in a chemical reaction. What happened to the rest? (purity, other reactions, remaining product, etc.)

Example 5: Consider the reaction between solutions of barium hydroxide and phosphoric acid, which forms water and quantitatively precipitates barium phosphate. A student mixes mL of M H 3 PO 4 with mL of M Ba(OH) 2 and collects 4.66 grams of a white precipitate. 1) Which reactant was the limiting reagent? 2) What was the theoretical amount of precipitate that should have been formed? 3) What is the percent yield for this experiment?

Solution Stoichiometry

Solutions  Uniform mixtures, composition is the same throughout  Solvent— substance doing the dissolving, present in a larger amount.  Solute— dissolved substance, present in a smaller amount  Examples: sugar in tea, salt water

Concentration  Amount of solute amount of solvent or solution  How much “stuff” is in a solution  Solute quantity in a certain amount of solvent/solution  Usually seen as mass percent value Ex. Orange juice, sweet tea

Molarity (M)  Main unit of concentration in chemistry  Moles solute L solution  Many solutions provide only the % solute by mass  need to know density to get molarity

Example 6: What is the concentration/molarity of a 50.0 ml solution with 1.25g of CuSO 4 in water?

Example 7: Luke Skywalker has a stock solution of aqueous formic acid that is 90% HCOOH by mass. The density of this solution is 1.20 g/ml. Skywalker wants to know the molarity of this solution to determine if it can be used in the construction of his latest project: starship destroyer X.

More Molarity Practice 1) A 500 ml solution of NaOH has a molarity of 0.50M. How many moles of NaOH are in the solution? 2) You have two solutions. One solution has 500 ml of 0.20 M NaCl and the other solution has 250 ml of 0.25 M NaCl.Which solution is more concentrated? 3) 2.75g of AgNO 3 dissolved in L of solution. Find the molarity. 4) How many grams of H 2 SO 4 are needed to make a 5M concentration solution with 250 ml?

Dilutions  Often want to prepare solutions of LESS concentration than stock solution  Technique of preparing a less concentrated solution by adding more solvent to decrease the solution’s concentration  Solute amount remains the same SOOO  M 1 V 1 = M 2 V 2  M 1 /V 1 values indicate initial “concentrated” solution  M 2 /V 2 values indicate final “dilute” solution

Example 8: Baking soda (NaHCO 3 ) neutralizes HCl in the following reaction when 175 ml of 1.55 M NaHCO 3 is added to 235 ml of 1.22 M HCl. NaHCO 3 (aq) + HCl (aq) NaCl (aq) + H 2 O (l) + CO 2 (g) a) How many grams of CO 2 are given off? b) What is the molarity of the aqueous NaCl solution?

Example 9:  Household bleach can be produced based on the chemical reaction below.  2NaOH (aq) + Cl 2 (g) NaOCl (aq) + NaCl (aq) + H 2 O (l)  a) What volume of 0.25M NaOH is required to produce 18g of NaOCl?  b) what volume of 0.25M NaOH is required to react with 0.32 moles of chlorine?  c) If 4.00L of 0.25M NaOH are used with excess chlorine gas, how much NaOCl is produced?

Homework:  Finish Stoichiometry Sheet  Percent Water Lab Report