CHAPTER 3B Stoichiometry Counting Atoms Amu – atomic mass unit 1 amu = 1.66 x 10-24 g Ex. 1 carbon atom = 12 amu 1 Fe atom = 55.85 amu 1 Na atom = ??? Amu Counting Atoms Calculate the mass, in amu, of a sample of Fe that contains 15 atoms. Calculate the mass, in amu, of a sample of carbon that contains 62 atoms. Counting Atoms Calculate the number of copper atoms present in a sample that has a mass of 1779.4 amu. 1 Cu atom = ??? Amu Calculate the number of argon atoms present in a sample that has a mass of 3755.3 amu. Moles An amount of matter that contains as many objects as the number of atoms in exactly 12 grams of pure carbon 12. This number is 6.02 x 1023. Avogadro's number = 6.02 x 1023 Moles 1 dozen eggs = 12 eggs 1 mole eggs = 6.02 x 1023 eggs 1 mole of elephants = 6.02 x 1023 elephants 1 mole of Al = 6.02 x 1023 Al atoms Moles But they all have different masses. 1 mol of C and 1 mol of Mg have the same number of particles but C has a mass of 12 amu and Mg a mass of 24 amu If C has a mass of 12 g by definition then Mg = 24 grams. So….. 1 mole of the element = 6.02 x 1023 atoms of that element and… 1 amu of an element = grams of 1 mole of element Element Al Au S Fe # of atoms 6.02 x 1023 6.02 x 1023 6.02 x 1023 6.02 x 1023 Mass (g) 26.98 196.97 32.07 55.85 Moles 1 mole 1 mole 1 mole 1 mole Mass/Mole 1 mol of an atom= average atomic mass of the element Ex. 1mol H = 1.008 g H 1mol C = 12.01 g C 1mol Na = 22.99 g Na Can then use this as a conversion factor Mass/Mole Compute the number of moles of atoms in a 25.0g sample of calcium. 1mol Ca = 40.08 g Mass/Mole Calculate the number of moles of atoms in 57.7 g sample of sulfur. Calculate the number of moles in 43.15 g of carbon. Calculate the mass in grams of 0.251 moles of sodium. Calculate the mass in grams of 5.58 moles of copper. Molar Mass The total mass of a compound. What is the mass of H20? 1 atom H2O = 1 atom O and 2 atoms H So… 1 mol H20 = 1 mol O and 2 mols H Mass of 1 mole of O = 1 x 16.00g Mass of 2 moles of H = 2 x 1.008g Mass of 1 mole of H2O = 16.00g = 2.016g = 18.12g Molar Mass What is the molar mass of C3H8? What is the molar mass of NH3? Molar Mass What is the mass in grams of 1.48 moles of potassium oxide? Calculate the mass of 4.85 mol of acetic acid, HC2H3O2. How many moles of formaldehyde (H2CO) does a 7.55 g sample represent? How many moles of tetraphosphorus dioxide does a 250.0 gram sample represent? Percent Mass Composition Determining the percentage of how much of the total weight each element weighs. Ex. Determine the mass percent of each element in sulfuric acid (H2SO4). %H= %S= %O= Percent Mass Composition Ex. Determine the mass percent of each element in C3H7OH Formulas Empirical formula – a formula of a compound that has the smallest whole-number ration of atoms present. Molecular formula – the actual formula of a compound. Ex. Determine the empirical formula for C4H10 H2 O 2 CCl4 Calculating Empirical Formulas 1. 2. 3. 4. Find mass of each element Number of moles of each element Divide each elements moles by the smallest number of moles. If the moles are not whole numbers, multiply each element by the number that converts the fraction to whole numbers Calculating Empirical Formulas Ex. A 1.500 gram sample of a compound containing only carbon and hydrogen is found to contain 1.198 gram of carbon. Determine the empirical formula for this compound. Calculating Empirical Formulas Ex. A sample of phosphoric acid contains 0.3086 grams of hydrogen, 3.161 grams of phosphorus, and 6.531 grams of oxygen. Determine the empirical formula for phosphoric acid. Calculating empirical Formulas Ex. The simplest amino acid, glycine, has the following mass percents: 32.00% carbon, 6.714% hydrogen, 42.63% oxygen, and 18.66% nitrogen. Determine the empirical formula for glycine. Mole Ratios A balanced equation of a reaction gives a ratio of moles of each compound involved in the reaction. Remember a mole has no dimension until you add one. Ex. NO + H2 → N2 + H2O Ex. SiH4 + NH3 → Si3N4 + H2 Mole Ratios Methane burns in oxygen to form carbon dioxide and water. CH4 + 2O2 → CO2 + 2H2O What number of moles of oxygen gas is required to react with 7.4 moles of methane? How many moles of carbon dioxide will be produced by reacting 2.6 moles of oxygen with excess methane? Mole Ratios Hydrogen sulfide gas reacts with oxygen to produce sulfur dioxide gas and water. How many moles of oxygen are required to react with 5.6 moles of hydrogen sulfide? How many moles of sulfur dioxide gas will be produced by reacting 7.3 moles of hydrogen sulfide with excess oxygen? How many moles of sulfur dioxide gas will be produced by reacting 7.3 moles of oxygen with excess hydrogen sulfide? Mole-Mass Conversion Solutions of sodium hydroxide absorb carbon dioxide from the air, forming sodium carbonate and water. Calculate the mass of carbon dioxide gas that is required to react with a solution containing 10.0 grams of sodium hydroxide. Calculate the mass of sodium carbonate that is produced when 10.0 grams of sodium hydroxide reacts with an excess of carbon dioxide. Mass/Mole Steps for calculating Balance equation Convert mass of known compound to moles. Use mole ratios from balanced equation as a conversion to moles of desired compound. Convert moles of desired compound to mass Limiting Reactants A reactant that stops the reaction because it is consumed. Steps to calculating limiting reactants Balance the equation. Convert to moles. Use the mole ratios to determine limiting reagent. Using limiting reactant compute the number moles of desired product. Convert to moles. Limiting reactants Iron(III) oxide reacts with carbon monoxide to form iron metal and carbon dioxide gas. In a certain experiment 5.0 grams of Iron (III) oxide is reacted with 5.0 grams of carbon dioxide. What mass of iron will be produced, assuming a complete reaction? What mass of carbon dioxide will be produced, assuming a complete reaction? Limiting reactants Iodine reacts with chlorine gas to form iodine chloride. In a certain experiment, 10.0 grams of iodine is reacted with 10.0 grams of chlorine gas. What mass of aluminum chloride will be produced, assuming a complete reaction? Percent yield actual yeild x100% Theoretical yeild In a certain experiment, the expected yield is 1.325 grams. The actual yield was 1.279 grams. Determine the percent yield. 2NH3 + CO2 → CN2H4O + H2O If 100.0 grams of ammonia is reacted with 100.0 grams of carbon dioxide and 120.0 grams of urea are produced. Determine the percent yield.