Stoichiometry-STEM PDF

STOICHIOMET RY Ms. Gina O. Tampol STOICHIOME the quantitative study of reactants and products in a TRY chemical reaction. “stochio” = Greek for element “metry” = measurement Mole Method the stoichiometric coefficients in...

STOICHIOMET RY Ms. Gina O. Tampol STOICHIOME the quantitative study of reactants and products in a TRY chemical reaction. “stochio” = Greek for element “metry” = measurement Mole Method the stoichiometric coefficients in a chemical equation can be interpreted as the number of moles of each substance AVOGADRO’S Italian scientist Amedeo Avogadro NUMBER mole (mol) mole (mol) is the amount of a substance that contains as many elementary entities (atoms, molecules, or other particles) molar mass (g/mol) the mass (in grams or kilograms) of 1 mole of units (such as atoms or molecules) of a substance. Molar mass of carbon- 12 (in grams) is numerically equal to its atomic mass in amu. atomic mass (sometimes called atomic weight) is the mass of the atom in atomic mass units (amu). Example: Zinc (Zn) is a silvery metal that is used in making brass (with copper) and in plating iron to prevent corrosion. How many moles of Zn are there in 23.3 g of Zn? Example: Sulfur (S) is a nonmetallic element that is present in coal. When coal is burned, sulfur is converted to sulfur dioxide and eventually to sulfuric acid that gives rise to the acid rain phenomenon. conversion factor: How many atoms are in 16.3 g of S? Avogadro’s number and conversion factor: Solution:  TRY: Silver (Ag) is a precious metal used mainly in jewelry. What is the mass (in grams) of one Ag atom? MOLECULAR MASS: (sometimes called molecular weight) is the sum of the atomic masses (in amu) in the molecule. Calculate the molecular masses (in amu) of the following compounds: (a) sulfur dioxide(SO2) (b) caffeine (C8H10N4O2) There are two O atoms and one S atom in SO2, so that: Calculate the molecular masses (in amu) of the following compounds: (a) sulfur dioxide(SO2) (b) caffeine (C8H10N4O2) There are eight C atoms, ten H atoms, four N atoms, and two O atoms in caffeine, so the molecular mass of C8H10N4O2 is given by MOLAR MASS: From the molecular mass we can determine the molar mass of a molecule or compound. The molar mass of a compound (in grams) is numerically equal to its molecular mass (in amu). Methane (CH4) is the principal component of natural gas. How many moles of CH4 are present in 6.07 g of CH4? conversion factor: Solution: there is 0.378 mole of CH4 in 6.07 g of CH4. PERCENT COMPOSITION BY MASS Percent Composition percent by mass of each element in a compound. PERCENT COMPOSITION BY MASS Phosphoric acid (H3PO4) is a colorless, syrupy liquid used in detergents, fertilizers,toothpastes, and in carbonated beverages for a “tangy” flavor. Calculate the percent composition by mass of H, P, and O in this compound. The molar mass of H3PO4 is 97.99 g. The percent by mass of each of the elements in H3PO4 is calculated as follows:  STOICHIOMETRY ( AMOUNTS OF REACTANTS AND PRODUCTS) STOICHIOMETRY STOICHIOMETRY Problem 1: The food we eat is degraded, or broken down, in our bodies to provide energy for growth and function. A general overall equation for this very complex process represents the degradation of glucose (C6H12O6) to carbon dioxide (CO2) and water (H2O): If 856 g of C6H12O6 is consumed by a person over a certain period, what is the mass of CO2 produced? Strategy: Looking at the balanced equation, how do we compare the amount of C6H12O6 and CO2? We can compare them based on the mole ratio from the balanced equation. Starting with grams of C6H12O6, how do we convert to moles of C6H12O6? Once moles of CO2 are determined using the mole ratio from the balanced equation how do we convert to grams of CO2? Problem 1: Step 1: The balanced equation is given in the problem. Step 2: Get the molar mass. To convert grams of C6H12O6 to moles of C6H12O6, we write Step 3: From the mole ratio, we see that 1 mol C6H12O6 ∞ 6 mol CO2. Therefore, the number of moles of CO2 formed is Problem 1: Step 4: Finally, the number of grams of CO2 formed is given by After some practice, we can combine the conversion steps Problem 1: Problem 2: All alkali metals react with water to produce hydrogen gas and the corresponding alkali metal hydroxide. A typical reaction is that between lithium and water: How many grams of Li are needed to produce 9.89 g of H2? Problem 2: The conversion steps are: Final answer: Problem 2: The conversion steps are: Final answer: Problem 3: How many moles of H2O are produced if 2.00 moles of O2 are used? Problem 4: How many grams of H2O are produced if 2.2 mol of NH3 are combined with excess oxygen? LIMITING REAGENTS reactant used up first in a reaction (because the maximum amount of product formed depends on how much of this reactant was originally present) Excess Reagents the reactants present in quantities greater than necessary to react with the quantity of the limiting reagent. Sample Problem: Consider the formation of nitrogen dioxide (NO2) from nitric oxide (NO) and oxygen: Suppose initially we have 8 moles of NO and 7 moles of O2. One way to determine which of the two reactants is the limiting reagent is to calculate the number of moles of NO2 obtained based on the initial quantities of NO and O2. From the preceding definition, we see that only the limiting reagent will yield the smaller amount of the product. Sample Problem: Because NO results in a smaller amount of NO2, it must be the limiting reagent. Therefore, O2 is the excess reagent. PROBLEM 1: Urea [(NH2)2CO] is prepared by reacting ammonia with carbon dioxide: In one process, 637.2 g of NH3 are treated with 1142 g of CO2. (a) Which of the two reactants is the limiting reagent? (b) Calculate the mass of (NH2)2CO formed. (c) How much excess reagent (in grams) is left at the end of the reaction?  RECALL: A Sulfur Dioxide reacts with Oxygen gas to form Sulfur Trioxide. a. If 3.4 moles of sulfur dioxide reacts with oxygen gas, how many moles of sulfur trioxide will form? b. How many moles of oxygen gas will react completely with 4.7 moles of sulfur dioxide. REACTION YIELD - the amount of product that would result if all the limiting reagent reacted. Theoretical Yield the maximum obtainable yield, predicted by the balanced equation. Actual Yield - the amount of product actually obtained from a reaction, - almost always less than the theoretical yield.. Percent Yield - describes the proportion of the actual yield to the theoretical yield. PROBLEM 1: Titanium is a strong, lightweight, corrosion-resistant metal that is used in rockets, aircraft, jet engines, and bicycle frames. It is prepared by the reaction of titanium(IV) chloride with molten magnesium between 950C and 1150C: In a certain industrial operation 3.54 107 g of TiCl4 are reacted with 1.13 107 g of Mg. (a)Calculate the theoretical yield of Ti in grams. (b)Calculate the percent yield if 7.91 106 g of Ti are actually obtained.  PROBLEM 1: In a certain industrial operation 3.54 x 10^7 g of TiCl4 are reacted with 1.13 x10^7 g of Mg. (a)Calculate the theoretical yield of Ti in grams. (b)Calculate the percent yield if 7.91 106 g of Ti are actually obtained. a. PROBLEM 1: a. PROBLEM 1: Note: The mass of Ti determined in part (a) is the theoretical yield. The amount given in part (b) is the actual yield of the reaction. b.  PROBLEM 2: When heated, lithium reacts with nitrogen to form lithium nitride: a. What is the theoretical yield of Li3N in grams when 12.3 g of Li are heated with 33.6 g of N2? b. If the actual yield of Li3N is 5.89 g, what is the percent yield of the reaction? THANK YOU