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Stoichiometry Problems

Stoichiometry Problems. How Matter is Quantified. 1. Volume – Gasses are usually quantified as volume. Covalent molecules that are a gas at room temperature and produce a pressure of 1 atm will occupy 22.4L each mole. 2. Molecules – All matter can be quantified as molecules.

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Stoichiometry Problems

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  1. Stoichiometry Problems

  2. How Matter is Quantified • 1. Volume – Gasses are usually quantified as volume. Covalent molecules that are a gas at room temperature and produce a pressure of 1 atm will occupy 22.4L each mole. • 2. Molecules – All matter can be quantified as molecules. • 3. Molarity – Aqueous solutions are quantified as molarity. It is how many moles that are dissolved in one liter of solution. • 4. Mass – All matter can be quantified as mass, however mass is used mostly for pure solids and liquids (not including solutions). When matter is quantified as mass, 1 gram of a substance will not have the same number of molecules as 1 gram of another substance, so it needs to be converted to moles. • 5. Moles – Matter is quantified as moles to make the quantity of molecules is a reasonably sized number. It is also the unit that unifies all other units.

  3. Converting Between Units

  4. Quantity Units • Sometimes your quantities are not in the proper unit and they must be converted to the proper unit before you can solve the problem • The quantity given will always be a metric quantity • Molarity - the unit is M (moles per liter) • If you are calculating the molarity from a volume that is in mL, you need to convert it to liters • Volume – the unit is L • If you are given a volume that is in mL, you need to convert your quantity to L • If you are asked to find a volume in mL, you need to convert your answer from L to mL • Mass – the unit is g • If you are given a mass that is in mg or kg, you need to convert your quantity to g • If you are asked to find the mass in mg or kg, you need to convert your answer from gto mg or kg

  5. Converting Sample Problems • How many moles are in 43g of SiO2? = 0.72 mol • The chart say to go from mass to moles, you need to divide by molar mass • The molar mass is calculated using a periodic table • It is the sum of the masses of each atom in the molecule • The molar mass of silicon dioxide is just Si + 2O

  6. Converting Sample Problems 2. What is the molarity of a 3.5L solution that contains 4.7 moles of NaCl? • Molarity– Another unit for moles. The unit is moles per liter. It tells you how many moles are in an aqueous solution. • The conversion chart says to convert moles into molarity, you need to divide the number of moles by the volume of the solution = 1.3M concentration 3. What is the volume at STP of 3 moles of oxygen? • STP – Standard temperature and pressure. This tells you the volume a gas would be when the pressure is 1 atm and the temperature is 0C (room temperature). • The conversion chart says that to convert moles into volume, you need to multiply by 22.4L 3 moles of O2 * 22.4L per mole = 67.2L of O2

  7. Answering a StoichiometryProblem • Stoichiometry - a branch of chemistry that deals with the relative quantity calculations of reactants and products. Example When dihydrogen sulfide is bubbled through a solution of lead (II) nitrate, a black precipitate of lead (II) sulfide forms as well as nitric acid. How many grams of lead (II) sulfide will be produced from 1.2 liters of a 2M solution of lead (II) nitrate. • The writing in red is a chemical calculation **The only quantity that will work when figuring out stoichiometry problems is moles. Whatever quantity you are given, you need to convert it to moles**

  8. Before you can solve a stoichiometry problem…. • 1. You must be able to find a balanced chemical equation • 2. You must be able to find a molar ratio from the balanced equation • 3. You must be able to convert between all units of matter • ANOTHER IMPORTANT NOTE: The only quantity that will work when figuring out stoichiometry problems is moles. Whatever quantity you are given, you need to convert it to moles, even if the question is asking for another unit. You find your answer in moles and convert to what the problem is asking for after you have your answer in moles.

  9. Molar ratio

  10. Molar ratio practice 2C3H8­O + 9O2 → 6CO2 + 8H2O Remember: The ratio always has what you are a trying to find on top and what you are given on the bottom. • What is the ratio of CO2 toO2? 6:9 • If I have 18 molecules of O2, how many molecules of CO2 can I make? 18 * 6/9 = 12 molecules • If I have 12 moles of O2, how many moles of CO2 can I make? 12 moles * 6/9 = 8 moles of CO2 • How many moles of O2 would be required to make 2 moles of CO2?= 2 * 9/6 = 3 moles of O2

  11. Back to the stoichiometry problem… Example When dihydrogen sulfide is bubbled through a solution of lead (II) nitrate, a black precipitate of lead (II) sulfide forms as well as nitric acid. How many grams of lead (II) sulfide will be produced from 1.2 liters of a 2M solution of lead (II) nitrate. • 1. You can’t find the molar ratio unless you have a balanced chemical equation. • 2. The unit given is molarity. You need to convert molarity to moles before you can multiply by the molar ratio. • 3. After you multiply by the molar ratio, your answer will be in moles, but the question is asking for grams so you need to convert to grams as your final answer.

  12. Steps to answer a stochiometry problem Step 1: Create a balanced equation to find the molar ratio. Step 2: Convert the quantity given to moles if it is in any other unit (Skip if it’s already in moles). Step 3: Multiply by the molar ratio to find the number of moles of what you are trying to find. Step 4: Convert the number of moles to the correct unit that the question is asking for (Skip if answer is supposed to be in moles).

  13. Solution Step 1: Balanced equation H2S + Pb(NO3)2 → PbS + 2HNO3 Step 2: Moles Pb(NO3)2 Molarity → Moles = 2 x 1.2 = 2.4 moles Step 3: Molar ratio • Looking for PbS from Pb(NO3)2 so the ratio is • 2.4 x 1 = 2.4 moles of lead (II) sulfide Step 4: Grams lead (II) sulfide • Molar mass PbS = 207 + 32 = 239 g/mol • 2.4 x 239 = 573.6g of PbS produced

  14. Quantity Units • Sometimes your quantities are not in the proper unit and they must be converted to the proper unit before you can solve the problem • The quantity given will always be a metric quantity • Molarity - the unit is M (moles per liter) • If you are calculating the molarity from a volume that is in mL, you need to convert it to liters • Volume – the unit is L • If you are given a volume that is in mL, you need to convert your quantity to L • If you are asked to find a volume in mL, you need to convert your answer from L to mL • Mass – the unit is g • If you are given a mass that is in mg or kg, you need to convert your quantity to g • If you are asked to find the mass in mg or kg, you need to convert your answer from gto mg or kg

  15. Limiting reactant • Limiting reactant/reagent - The reactant that limits the quantity produced. It is the reactant that you will run out of first. EXAMPLE How many grams of copper (II) phospate will be produced from 10 mL of a 2M solution of sodium phosphate and 10 mL of a 0.238M solution of copper (II) chloride? **You can easily identify a limiting reactant problem because it has 2 quantities in the problem (your 2 reactants)**

  16. Before you begin Are all of your units correct? How many grams of copper (II) phosphate will be produced from 10 mL of a 2M solution of sodium phosphate and 10 mL of a 0.238M solution of copper (II) chloride? Both volumes are in mL!!! What is the proper amount of each reactant in the correct unit? LITERS!!!!

  17. Finding the limiting reactant How many grams of copper (II) phosphate will be produced from 10 mL of a 2M solution of sodium phosphate and 10 mL of a 0.238M solution of copper (II) chloride? Step 1: Balanced chemical equation: 2Na3PO4 + 3CuCl2 6NaCl + Cu3(PO4)2 Step 2: Convert the quantities given to moles: 0.01L * 2M = 0.02 moles Na3PO4 0.01L *0.238M = 0.00238 moles CuCl2

  18. Finding the limiting reactant Step 3 - Divide the number of moles of each reactant by their coefficient. Whichever is the smaller number is the limiting reagent. 2Na3PO4 + 3CuCl2 6NaCl + Cu3(PO4)2 = 0.01 Therefore, this reaction can be done 0.01(6x1023) times (the larger number) = 0.00079 Therefore, this reaction can be done 0.00079(6x1023) times (the smaller number)

  19. Finding the quantity Produced 2Na3PO4 + 3CuCl2 6NaCl + Cu3(PO4)2 Step 4 – multiply your limiting quantity by the molar ratio that you get from the balanced chemical equation to get the moles of product 0.00238 moles CuCl2x = 0.00079 moles Cu3(PO4)2 Step 5 – since your answer is in moles and the question is asking for grams, you need to convert to grams 0.000793 * (2x64 + 2x31 + 8x16) = 0.251g Cu3(PO4)2

  20. Percent yield • After you’ve calculated the amount of sample being produced and you go ahead and perform the reaction, the actual amount being produced will not be 100% accurate, which is why you must measure the accuracy of your calculation. • Percent yield is calculated as follows:

  21. Percent yield example • The theoretical yield from the previous problem was determined to be 0.251g of Cu3(PO4)2 • After doing the experiment, the actual yield was 0.183g of Cu3(PO4)2 • What is the theoretical yield? 0.183g/0.251g = 73% yield

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