Unit VI Stoichiometry Applying Mole Town to Reactions
Learning Goals I can apply mole town to reactions to determine the amount of product based on the amount of a reactant. I can apply mole town to reaction to determine the amount of reactant needed to produce an amount product. I can calculate the molarity of a solution. I can determine how much of a known solution will be needed to make a certain amount of another solution. I can determine the ph of an unknown acid or base based upon the amount of a known acid or base that is necessary to neutralize it. I can determine the limiting reagent in a reaction and how much product will be produced based on that limiting reagent.
Stoichiometry The study of quantities of materials consumed and produced in chemical reactions. Simply the math behind chemistry. Given enough information, one can use stoichiometry to calculate masses, moles, and percents within a chemical equation.
Stoichiometry and Mole Town Stoichiometry adds a step to Mole Town When doing calculations to Mole Town, you have to convert from one substituent in a reaction to another in the same reaction Steps 1. Make sure the reaction you are using is balanced. 2. From wherever you are, get into moles (like before). 3. Switch between compounds using their molar ratios. 4. Get out of moles, if necessary to new unit of new substituent.
Examples 23g Na 22.99g Na If 23 grams of Na are reacted with excess Cl 2, how many grams of NaCl will be produced? 1. Balance the equation Na + Cl 2 NaCl 2Na + Cl 2 2NaCl 2. Grams moles 3. Switch between compounds 1. 2 NaCl : 2 Na 4. Get back out of moles, into grams = (23 x 1 x 2 x 48.44) / (22.99 x 2 x 1) = 48.65 g of NaCl 1 mole Na 2 mole NaCl 2 mole Na 48.44g NaCl 1mole NaCl
Example 2 If 44.8L of O 2 are reacted with excess H 2, how many moles of H 2 O will be produced? H 2 + O 2 H 2 O
Molarity Most of chemistry happens in solution Molarity is a measurement of concentration of a solution The amount of a solute (the stuff being dissolved) per liter of solution M = moles of a solute /liter of solution
Molarity without Stoichiometry Determine the moles of the solute (if necessary) Divide by the volume Example: What is the molarity of 1.75 moles of solute dissolved in 5.26L of solvent?
Molarity Examples What is the molarity of a solution when 2.78 moles of solute are dissolved in 265.2 ml of solvent? How many grams of NaCl should be used to prepare a 3.3L solution of saltwater with the molarity of 4.26 M?
Molarity with Stoichiometry When using stoich to calculate molarity, always start with the compound that the question is NOT about. 1. Balance the equation 2. Start with the known compound Example: What mass of NaHCO 3 is necessary to neutralize 25.0L of 2.0M of HC 2 H 3 O 2? NaHCO 3 + HC 2 H 3 O 2 NaC 2 H 3 O 2 + H 2 O + CO 2 = 4205g NaHCO 3 25 L HC 2 H 3 O 2 2.0 mol HC 2 H 3 O 2 1.0 mol NaHCO 3 84.01 g NaHCO 3 = 25 x 2.0 x 1.0 x 84.01g 1 L HC 2 H 3 O 2 1.0 mol HC 2 H 3 O 2 1.0 mol NaHCO 3 = 1 x 1.0 x 1.0
Example How many milliliters of 0.8M of acetic acid are necessary to produce 450mL of carbon dioxide gas at STP?
Example It took 10.0grams of sodium bicarbonate to completely neutralize 15.0mL of acetic acid. What was the molarity of the acid?
Warm Up How many milliliters of 0.6M hydrochloric acid are needed to produce 60g of sodium chloride? HCl + NaOH NaCl + H 2 O
Dilutions A solution with a known concentration can be diluted with water to prepare a solution with any desired concentration that is lower than the original solution. M 1 V 1 = M 2 V 2 M 1 = the molarity of the original solution V 1 = the volume of the original solution M 2 = the molarity of the new solution V 2 = the volume of the new solution
Example Calculate the number of milliliters of concentrated hydrochloric acid, 12.0M HCl, that must be used to prepare 500. ml of 2.00M HCl solution. Known? Unknown? M 1 = 12.0M M 2 = 2.00M V 2 = 500mL V 1 =? M 1 V 1 = M 2 V 2 12.0(V 1 ) = 2.00(500mL) V 1 = (1000 M x ml) / 12.0M V 1 = 83.3 ml
Example Determine the number of milliliters of a 1.20M concentrated hydrochloric acid solution needed to prepare 2.00L of a 0.100M solution.
Example You wish to prepare 5.1M H 2 SO 4 from stock solution of sulfuric acid that is 18M. How many milliliters of stock solution are necessary to create 257mL of the 5.1M H 2 SO 4?
Titrations REMEMBER: Acids release H + when they dissolve and bases release OH -. When acids and bases react with each other, they neutralize. They create water and a salt. Acids have a ph below 7 and bases have a ph above 7. ph measure the concentration of H + ions in solution. Water has a ph of 7
Titrations Neutralizing an acid with a base to determine the concentration of one or the other By knowing the concentration (molarity) of one of the substances, you can calculate the molarity of the other by doing a titration This process is bringing it all together Stoichiometry (ratios of reactions) Dilutions (molarity) HALT PLEASE have patience with yourself and the process (and me ). I PROMISE it will come if you TRY!!!!!!!!!!!! Understanding something will be never come by whining about how hard it is!!!
Titration Calculation Steps 1. Write down everything you know. 2. Write the reaction (or as much as you know) Make sure the reaction is balanced 3. Check all your info for consistency All volumes should be in L not ml, etc 4. Calculate the number of moles of the reactant you know the most about 5. From the balanced equation, determine the mole ratios 6. Calculate the number of moles of the unknown. 7. Using the volume of the unknown and moles of unknown, calculate the Molarity
Titrations Helpful Hints Step One Read the question carefully and ask yourself, What is the question giving me and what is it asking for? NEXT.. Figure out what tools you have to get from one thing to the other? Next read the question again!!
Example - 10.0mL of HCl (hydrochloric acid) are titrated with 15.0mL of 2.0M sodium hydroxide (NaOH). What is the molarity of the acid? 2.0 moles NaOH 1. Known? V HCl = 10.0mL V NaOH = 15.0mL M NaOH = 2.0M 2. Reaction? HCl + NaOH NaCl + H 2 O 3. ml L V HCl = 10.0mL = 0.0100 L V NaOH = 15.0mL = 0.0150 L 4. Moles of known 0.0150 L NaOH 1 mol HCl reactant 5. Mole ratios 1:1 6. Moles of Unknown? 7. Volume of unknown molarity of unknown = 2.0 x 0.0150 x 1 mol = 1 x 1 x 0.0100 L = 0.03 / 0.01 = 3M 1L NaOH 1 mol NaOH 0.0100 L HCl
Example If 25.6mL of 4.40M NaOH was required to neutralize 325mL of H 2 SO 4, what was the molarity of the H 2 SO 4 solution?
Example Calculate the molarity of a NaOH solution if a 31.22mL sample of it requires 12.53 ml of 0.100M H 2 SO 4 to neutralize the base.
Warm Up If 25mL of 2.5M HCl are needed to neutralize 78mL of an unknown NaOH solution, what is the molarity of the NaOH?
Limiting Reagent In a reaction, the reactant that limits the amount of product formed is considered the limiting reagent. That reactant must be completely used up and the other reactant will be in excess Example: How many hamburgers can I make with 6 pieces of bread and 6 hamburger patties? When I consider just the bread 6 slices 1 burger = 6 burgers = 3 burgers 2 slices = 2 When I consider just the patties 6 patties 1 burger = 6 burgers = 1 burgers 1 patty = 1 The bread is the limiting reagent
Limiting Reagent Problems 1. You ll be given 2 starting amounts. 2. Figure out how much of the desired product can be formed in the right unit. 3. The limiting reagent limits the amount of product that can be formed. 4. The answer is the minimum amount of product formed!!
Example - Large quantities of ammonia, NH3, are burned in the presence of a platinum catalyst to give nitrogen monoxide. Suppose a vessel contains 2.20 moles of NH 3 and 1.20 moles of O 2. Which is the limiting reagent? How many moles of NO could be obtained? 4 NH 3 + 5O 2 4 NO + 6 H 2 O 1. Known? 2.20 Moles of NH 3 1.20 moles of O 2 How much product? 2.20 mol NH 3 4 mol NO 4 mol NH 3 = 2.20 mol NO Limiting reagent? O 2 How much NO? 0.96 mol 1.20 mol NH 3 4 mol NO 5 mol NH 3 = 0.96 mol NO
Example Carbon disulfide, CS 2, burns in oxygen. How many liters of CO 2 are there produced when 23.65 grams of CS 2 and 32.34 grams of O 2 react?
Warm Up If 25.2g of CO 2 react with water, how much H 2 CO 3 will be produced? CO 2 + H 2 O H 2 CO 3 How many grams of NaCl are required to produce 500mL of 2.2M solution? How much stock solution, 15M, is necessary to produce 500mL of 2.5M solution? If 50mL of NaOH are required to neutralize 27mL of 0.5M HCl, what is the molarity of the NaOH? HCl + NaOH H 2 O + NaCl