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Stoichiometry and the Mole

Stoichiometry and the Mole. The Scenario. You are planning to do a reaction using hydrochloric acid. Suddenly, you drop the bottle and spill all 13grams of your very dangerous acid on the floor!

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Stoichiometry and the Mole

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  1. Stoichiometry and the Mole

  2. The Scenario • You are planning to do a reaction using hydrochloric acid. Suddenly, you drop the bottle and spill all 13grams of your very dangerous acid on the floor! • You know it can be neutralized (so that it stops eating through the floor) by sodium hydroxide, but you need to know how much sodium hydroxide you need to completely get rid of all of the acid. How do you do this??????

  3. The acid spill… • You know that: • HCl + NaOH ----> NaCl + H2O • Very dangerous ---> Very harmless • You also know you have 13 g of HCl • Wouldn’t it be nice to know how much one atom of an element weighed! • The big problem is that elements have different isotopes.

  4. Isotopes? • What is an isotope? • Atoms of the same element with different numbers of neutrons. • Ex. Hydrogen-1, Hydrogen-2, Hydrogen-3 • 1H, 2H, 3H

  5. Average Atomic Mass • The average atomic mass for all elements are shown on the periodic table. • The periodic table shows the AVERAGE weights of all isotopes.

  6. Atomic Mass Units • All masses for atoms on the periodic table are in amu’s. • 1 amu = 1.66 x 10-24g

  7. Formula Mass (Weight)Molecular Mass (Weight) • Formula mass (aka molecular weight) is the mass of one molecule. • Formula mass = atomic masses of elements x subscripts

  8. Find the formula mass of water. • Water is H2O • From periodic table H --> 1.008 amu’s • From periodic table O --> 16.00 amu’s • BUT there are 2 H’s and only 1 O in H2O • Solution: • H --> 2 x 1.008 = 2.016 amu • O --> 1 x 16.00 = 16.00 amu + Add these together don't forget sig figs! 18.02 amu

  9. Find the formula masses for the following: • CO2 • K2CrO4 • NH4 • (NH4)3PO4 • H3PO4 44.01 amu 194.2 amu 18.04 amu 149.1 amu 97.99 amu

  10. Percent Composition Percent composition is the percent by weight of each elementin a compound. Step 1: Find the formula mass of the compound.. Step 2: % Composition = Total mass of element X 100 Formula mass of cmpd

  11. Percent Composition • What percentage of the total mass is taken up by one type of atom? • % comp = [mass of atoms in questions] [formula mass] • What percentage of total mass do EACH of the types of atoms in water contribute? • Hint: Water has 2 types of atoms, hydrogens and oxygens, so you are doing 2 separate problems here. X 100

  12. Percent Comp. Of Water • 1. For each element, multiply atomic mass by subscript: • H2 2 x 1.008 = 2.016 • O 1 x 16.00 = 16.00 • 2. Find formula mass: • 2.016 + 16.00 = 18.02 amu • 3. Plug in to percent composition for each element present. • H --> [2.016] / [18.02] = 11.19 % • O --> [16.00 / 18.02] = 88.79 % • Notice they add up to nearly 100% (only nearly because of error)

  13. What is percent comp. For OXYGEN in each of the following? • CO2 • K2CrO4 • NH4 • (NH4)3PO4 • H3PO4 72.71% 32.96% 0% (no oxygen present) 42.92 % 65.31 %

  14. A Mass Challenge • 1. How many grams of oxygen are in 237g of H3PO4? • 2. How many grams of hydrogen are in 360g of (NH4)3PO4? • 3. How many grams of copper are in 296g of Cu(OH)2?

  15. Introducing…The Mole!!! • 1 mole = approximately 602,204,531,000,000,000,000,000 • That is… • 602 sextillion • 204 quintillion • 531 quadrillion • plus or minus a few hundred trillion

  16. The Mole • 1 mole = 6.022 x 1023 particles. • A mole is the “unit if measure. • 6.02 x 1023 is called Avogadro’s Number.

  17. Avogadro’s Number No! Not avocado - Avogadro!

  18. Mole Conversions • How much does one mole of hydrogen atoms weigh in grams? • Recall: 1 amu = 1.66 x 10-24g So, 1 atom of H weighs 1.008 amu AND 1 mole of H atoms weighs 1.008 g

  19. Conversion options with the mole • 1 mole = 6.022 x 1023 particles • 1 mole of atoms = mass on per. table in grams • 1 mole = 22.4 L (at STP & for gases only!!) STP is standard Temperature (273 K) & Pressure (1 atm)

  20. Molar Mass (aka Gram formula Mass) 1 mole CO2 = 6.02 x 1023 molecules CO2 1 mole CO2 = 22.4 L CO2 if at STP How many grams does 1 mole of CO2 weigh? C = 1 x 12.01 = 12.01g O= 2 x 16.00 = 32.00g + 44 . 01 g / mole of CO2

  21. Molar Mass Practice • 1. What is the gram formula mass for K2CrO4? • 2. Molar mass for (NH4)3PO4? 149.12 g/ mole (NH4)3PO4

  22. Volume and the Mole • 1. How many moles are in 38L of carbon dioxide at STP? • 2. How many liters are in 6.4moles of oxygen at STP?

  23. Particles and the Mole • 1. 1.20 x 1025 atoms of P = How many moles of P? • 2. 0.750 moles Zn = How many atoms of Zn? • 3. How many moles are in 9.2x 1020 molecules of CO2?

  24. Molevillian Islands

  25. Mole Conversions Practice • 4. How many grams are in 9.2 x 1020 molecules of CO2? • 5. How many grams are in 57.2 L of hydrogen gas at STP?

  26. Stoichiometryopolis Molewian Islands Molevillian Islands

  27. So what is “MoleAmerican Airlines” Representing? • Interpreting Chemical Equations • What does a chemical equation ACTUALLY tell you? What does it all mean? • Ex: N2(g) + 3H2(g) ---> 2NH3(g) • This says that 1 molecule of N2 reacts with 3 molecules of H2 to give 2 molecules of NH3 gas. • Let’s say you have 1 mole of nitrogen gas and an unlimited supply of hydrogen gas • 1. How much of the hydrogen gas will react with the nitrogen gas? • 2. How much ammonia gas can you make? 3 moles 2 moles

  28. I. Intro to Stoichiometry Stoichiometry is the process of calculating the amount of substances produced in a chemical reaction. When you know the amount of one substance, you can determine the amount of the other reactants or products.

  29. II. Mole Ratios N2 + 3H2 2NH3 Coefficient represent the number of moles of that substance. Mole ratios are written in fraction form.

  30. N2 + 3H2 2NH3 • Mole ratios: 3 mol H2 1 mol N2 Left side Reactants: 1 mol N2 3 mol H2 or Right side Products: 1 mol N2 2 mol NH3 2 mol NH3 1 mol N2 or Right side Products: 3 mol H2 2 mol NH3 or 2 mol NH3 3 mol H2

  31. Mole Road Map Grams A Moles A Moles B Grams B Use Mole Ratio Use Molar Mass Use Molar Mass

  32. IV. Mole to Mole Stoichiometry (Moles A to Moles B on roadmap) Ex. Determine the number of oxygen moles required to burn 1.20 moles of ethyl alcohol, C2H5OH. C2H5OH + 3O2 2 CO2 + 3H2O 1.2 mol A ?mol B Step 1: Balance equation. (This is already balanced!) Step 2: Write the given amount of the chemical – this is your “moles A.” Step 3: Use the mole ratio from the balanced equation. 1.20 mol C2H5OH 3 mol O2 1 mol C2H5OH = 3.60 mol O2 Answer

  33. Mole Road Map Grams A Moles A Moles B Grams B Use Mole Ratio Use Molar Mass Use Molar Mass

  34. V. Mole to Mass Stoichiometry (Moles A to Grams B on roadmap – Just one more step to the problem we just did!) Ex. Determine the grams of oxygen required to burn 1.20 mol of ethyl alcohol, C2H5OH. Step 1: Write a complete balanced equation. Step 2: Write the given amount. Step 3: According to the mole road map, you need to go from: Moles A  Moles B  Grams B 1.20 mol C2H5OH 3 mol O2 32 g O2 1 mol C2H5OH 1 mol O2 = 115 g O2 Answer

  35. Mole-Mole Problems • Balanced chemical equations, through their coefficients indicate the ratio of moles of reactants and products. • 1. Aluminum and oxygen react to form aluminum oxide. If you have 2.3 moles of aluminum oxide after the reaction how many moles of aluminum were used? • STEP 1: Write the balances equation: • 4Al + 3O2 ---> 2Al2O3

  36. Mole-Mole Problem cont. • STEP 2: Do dimensional analysis starting with what you know to convert moles of aluminum oxide to moles of aluminum. • Aluminum and oxygen react to form aluminum oxide. If you have 2.3 moles of aluminum oxide after the reaction how many moles of aluminum were used? • 4Al + 3O2 ---> 2Al2O3

  37. Now you try a mole-mole problem • 2. Hydrogen gas and oxygen gas react to make water. If you have 3.24 moles of hydrogen gas and an unlimited supply of oxygen gas, how many moles of water can you make? • H2 + O2 ---> 2H2O

  38. Mole-Mass Problems • You have 2.4 moles of Al reacting with oxygen. How many grams of Al2O3 are produced? • STEP 1: Write your balanced equation • 4Al + 3O2 ---> 2Al2O3 • STEP 2: Do dimensional analysis starting with what you know to convert moles of aluminum oxide to moles of aluminum.

  39. Mole-Mass Problems- you try: • You have 6.7 moles of oxygen gas reacting with unlimited supply of hydrogen. How many grams of water are produced? • H2 + O2 ---> 2H2O

  40. Mole Road Map Grams A Moles A Moles B Grams B Use Mole Ratio Use Molar Mass Use Molar Mass

  41. Mass-Mass Problems • Dicarbon Dihydride reacts with oxygen gas to produce carbon dioxide and water. How many grams of oxygen gas will react completely with 13.0g of C2H2? • STEP 1: Write the balanced equation. • 2C2H2 + 5O2 ------> 4CO2 + 2H2O • STEP 2: Do dimensional analysis.

  42. You try: Mass-Mass Problem • CaC2 reacts with water to produce C2H2 and calcium hydroxide. If 51.6g of CaC2 are reacted, how many grams of calcium hydroxide are produced? • CaC2 + 2H2O ---> C2H2 + Ca(OH)2

  43. Theoretical, Experimental, & Percent Yields Theoretical yield is the amount of product that you mathematically determine (using stoich) from a chemical reaction. Experimental yield is the amount of product that you actually get when you perform the reaction in lab. Percent yield = Experimental Yield x 100 Theoretical Yield

  44. Calculating Chemical Formulas • Empirical Formulas: Simplest whole number ratio of atoms in a compound • Ex: C6H12O6 is a molecular formula. The empirical formula is CH2O.

  45. Determining Empirical Formulas • Step One:Convert grams to moles. • Step Two: Divide each by the smallest number of moles to get subscripts • Step Three: If you do not have a whole number,or one that is reasonably close, multiply by an integer to obtain a whole number.

  46. Empirical Formula / . 337 = 1 • Hg--> 67.6g x 1mole = .337 moles Hg 200.59g • S --> 10.8g x 1mole = .337 moles S 32.06g • O --> 21.6g x 1mole = 1.35 moles O 16.00g Now divide each by the smallest number of moles / . 337 = 1 / . 337 = 4 HgSO4 These are your subscripts for your formula!

  47. Empirical Formula from %cont… • Problem 2: After doing several experiments on an unknown substance it was found that the percent compositions for the compound were: 3.09% H, 31.6% P, and 65.3 % O. What is the empirical formula for this substance? H3PO4

  48. Empirical Formulas if given mass. • STEP 1: Turn mass into moles • STEP 2: Divide by lowest number of moles to get subscripts. • Problem 1: A 19.12 gram sample of an unknown chemical was discovered. After much experimentation it was determined that the unknown was made of 3.03g of hydrogen and 16.09g of oxygen. What is the empirical formula? H3O

  49. Molecular Formulas • This is the exact formula of the molecule giving types of atoms and numbers of each type. This formula represents how the compound actually appears in nature. • You will need to be able to determine the molecular formula using data.

  50. Determining Molecular Formula • Example problem: A compound has a molar mass (gfm) of 120.00 g/mole with an empirical formula of CH2O. What is the molecular formula? • STEP ONE: Find the molar mass (gfm) of the empirical formula • STEP TWO: Divide true molar mass (gfm) by gfm of the empirical formula. • STEP THREE: Write the molecular formula by multiplying subscripts by answer to step 2. 120.00 / 30.03 = 3.996 ~ 4 C4H8O4 C[1x4]H[2x4]O[1x4]

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