AP Chemistry Unit #3 Chapter 3 Zumdahl Stoichiometry C6H12O6 + 6 O2 6 CO2 + 6 H2O Students should be able to: Calculate the atomic weight (average atomic mass) of an element from the relative abundances and masses of its naturally occurring isotopes. Calculate the percentage composition of a compound form its formula. Calculate the molar mass of a substance from its chemical formula. Be able to interconvert between moles, mass, and number of particles of a substance. Calculate the empirical formula of a compound from either elemental percent composition or quantity of CO 2 and H 2 O produced from its combustion. Calculate the molecular formula of a compound from the empirical formula and molecular weight. Find the mass of any substance in a chemical reaction from the mass of one substance. Determine the limiting reactant (limiting reagent) in a reaction and then calculate the amount of each product and the mass of the excess reactant left over. Calculate theoretical yield. Keywords: atomic mass atomic mass unit atomic weight (average atomic mass) formula weight molecular weight mole Avogadro's number molar mass empirical formula molecular formula limiting reactant (limiting percent yield reagent)
I. Chemical Equations A. Chemical Equations A representation of a chemical rxn that uses chemical formulas of the reactants and products. Example) 2 H 2 (g) + O 2 (g) 2 H 2 O (g) Coefficient B. Balancing Chemical Equations The Law of Conservation of Matter - A Balanced Chemical Equation has equal numbers of atoms on both sides of the equation, and thus equal mass. - Equations are balanced by adding coefficients in front of the appropriate chemical formulas. C. Sample Exercise 3.1 Balance the following equation: Na (s) + H 2 O (l) NaOH (aq) + H 2 (g) C 2 H 4 + O 2 CO 2 + H 2 O II. Patterns of Chemical Reactivity A. How can you use the Periodic Table to predict Chemical Reactions? - In general, elements in the same family have similar chemical reactivity, and therefore undergo similar chemical reactions. 1. What happens when an alkali metal reacts with water? 2 M (s) + 2 H 2 O (l) 2 MOH (aq) + H 2 (g) alkali metal + water metal hydroxide + hydrogen gas 2. Predict what would happen if an alkaline earth metal reacts with water: M (s) + 2 H 2 O (l) M(OH) 2 (aq) + H 2 (g) Alkaline earth metal + water metal hydroxide + hydrogen gas B. What is a Combustion Reaction? Rapid reactions that produce a flame. CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g) 2
C. Sample Exercise 3.2 Write the balanced chemical equation for the reaction that occurs when menthanol, CH 3 OH (l), is burned in air. D. What is a Combination Reaction? 2 or more substances react to form 1 product (AKA Synthesis rxn). 1. Example 2 Mg(s) + O 2 (g) 2 MgO (s) E. What is a Decomposition Reaction? 1 substance breaks-down to produce 2 or more other substances. 1. Example CaCO 3 (s) CaO (s) + CO 2 (g) III. Atomic and Molecular Weights A. The Atomic Mass Scale Developed from the law of definite proportions (constant compositions) 1. Atomic Mass Unit ( amu ) the mass of 1 proton (and 1 neutron) = 1 amu. 2. What is the Atomic Mass Unit based on? The scale is based on the standard Carbon 12 atom. B. Average Atomic Masses Periodic Tables report out atomic mass (a.k.a. atomic weight) it is the average mass of all the naturally occurring isotopes for a particular element. 1. What is the difference between mass number and average atomic mass / atomic weight? Mass # refers to one specific isotope 3
C. Sample Exercise 3.3 Naturally occurring chlorine is 75.33% Cl-35, which has an atomic mass of 34.969 amu, and 24.47% Cl-37, which has an atomic mass of 36.966. Calculate the average atomic mass (atomic weight) of chlorine. D. Formula and Molecular Weights Aka - Formula mass / molecular mass / gram formula mass - the sum of the atomic weights of each atom in a chemical formula. - The mass of 1 mole of any substance is equal to its formula mass expressed in grams. E. Sample Exercise 3.4 Calculate the formula weight of (a) sucrose, C 12 H 22 O 11 ; (b) calcium nitrate, Ca(NO 3 ) 2. F. What is a Mass Spectrometer? an instrument that accurately determines that molecular weight of a substance by ionizing a gaseous sample and measuring the degree of deflection through a magnetic field. 1. How does one work? The more massive the ion, the lesser the deflection detected by the instrument. 4
G. % Composition from formulas The % by mass contributed by each element in a substance. % Comp = mass of the part x 100% mass of the whole 1. Sample Exercise 3.5 Calculate the % composition of C 12 H 22 O 11. IV. The MOLE A. The Definition a unit of measure that equals 6.02 x 10 23 particles. B. Avogadro s Number 6.02 x 10 23 1. Sample Exercise 3.6 How many C atoms are in 0.350 mol of C 6 H 12 O 6? C. Molar Mass The mass of 1 mole of any substance is equal to its gram formula mass. 1. Sample Exercise 3.7 What is the mass of 1 mol of glucose, C 6 H 12 O 6? 5
D. Converting between masses, moles, and # of particles: 1. Sample Exercise 3.8 How many moles of glucose, C 6 H 12 O 6, are in (a) 538g and (b) 1.00g of this substance? 2. Sample Exercise 3.9 - What is the mass, in grams, of 0.433 mol of Ca(NO 3 ) 2? 3. Sample Exercise 3.10 How many glucose molecules are in 5.23 g of C 6 H 12 O 6? 6
V. Empirical Formulas from Analyses A. Definition B. Sample Exercise 3.11 Ascorbic Acid (Vitamin C) contains 40.92% C, 4.58% H, and 54.50% O by mass. What is the empirical formula of ascorbic acid? C. Finding the Molecular Formula from the Empirical Formula D. Sample Exercise 3.12 Mesitylene, a hydrocarbon that occurs in small amounts in crude oil, has an empirical formula of C 3 H 4. The experimentally determined molecular weight of this substance is 121 amu. What is the molecular formula of mesitylene? 7
E. Combustion Analysis 1. Example Problem An unknown substance is known to contain only C, H, and O. Combustion of 0.255 g of the substance produces 0.561 g CO 2 and 0.306 g H 2 O. Calculate the empirical formula of the substance. VI. Quantitative Info from Balanced Equations (Stoichiometry) A. What do the coefficients in a balanced chemical equation tell us? The coefficients in a balanced chemical equation can be interpreted both as the relative # of molecules (or formula units) involved in the rxn and as the relative # of moles. B. How do we solve mass mass problems? Grams of Substance A Grams of Substance B MOLES of Substance A MOLES of Substance B 8
C. Sample Exercise 3.13 How many grams of water are produced in the combustion of 1.00 g of glucose, C 6 H 12 O 6? D. Sample Exercise 3.14 Solid lithium hydroxide is used in space vehicles to remove exhaled CO 2. The lithium hydroxide reacts with the gaseous CO 2 to form solid lithium carbonate and liquid water. How many grams of CO 2 can be absorbed by each 1.00 g of lithium hydroxide? 9
VII. Limiting Reactants A. What is a Limiting Reactant (Limiting Reagent)? If during a rxn the reactants are mixed without using the stoichiometric proportions, 1 of the reactants may be totally consumed. The Limiting Reactant is the reactant that is completely consumed during a reaction. B. Sample Exercise 3.15 Part of the SO 2 that is introduced into the atmosphere ends up being converted to sulfuric acid. The net reaction is : 2 SO 2 (g) + O 2 (g) + 2H 2 O (l) 2 H 2 SO 4 (aq) How much sulfuric acid can be formed from 5.0 mol of SO 2, 1.0 mol of O 2, and an unlimited quantity of H 2 O? C. Sample Exercise 3.16 Consider the following reaction: 2 Na 3 PO 4 (aq) + 3 Ba(NO 3 ) 2 (aq) Ba 3 (PO 4 ) 2 (s) + 6 NaNO 3 (aq) Suppose that a solution containing 3.50 g of Na 3 PO 4 is mixed with a solution containing 6.40 g of Ba(NO 3 ) 2. How many grams of Ba 3 (PO 4 ) 2 can be formed? 10
D. What is a theoretical yield? Is the quantity of product that is calculated to form when the entire limiting reagent is completely reacted. - The amount of product actually obtained during the rxn is called the actual yield. 1. What is the percent yield? Compares the theoretical yield and the actual yield. % yield = Actual Yield x 100% Theoretical Yield E. Sample Exercise 3.17 Adipic acid, H 2 C 6 H 8 O 4, is a raw material used for the production of nylon. It is made commercially by a controlled reaction between cyclohexane, C 6 H 12, and O 2 : 2 C 6 H 12 + 5 O 2 2 H 2 C 6 H 8 O 4 + 2 H 2 O (a) (b) Assume that you carry out this reaction starting with 25.0 g of cyclohexane, and the cyclohexane is the limiting reactant. What is the theoretical yield of adipic acid? If you obtain 33.5 g of adipic acid for your reaction, what is the percent yield of adipic acid? 11