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# chapter%203 - CHM 25 – Spring ‘09 Prof R.S Miller...

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Unformatted text preview: CHM 25 – Spring ‘09 Prof. R.S. Miller Miller Chapter 3 Mole concept Mole concept Mole – purposely defined as amount of atoms in exactly 12 g of pure C-12. abbreviation - mol Number of individual entities is called Avogadro’s number (N number (NA )= 6.022 x 1023 mol-1 10 Molar mass (M )– mass of 1 mole of a substance. 1 mol C-12 = 6.02 x 1023 C-12 atoms = 12.000g 1 mol C atoms = 6.02 x 1023 C atoms = 12.011 g 1 mol F atoms = 6.02 x 1023 F atoms = 18.998 g One One-Mole Amounts Amounts Moles of an element – exceptions: H2, O2, N2, F2, Cl2, Br2, I2 – diatomic molecules molecules Others - P4 & S8 Using moles in calculations Conversion factors: 1 mol C 6.02 x 1023 C atoms 10 atoms And 1 mol mol 12.011 g C Use units to guide you in calculations calculations Example: Determine the molar mass, M of an element if the mass of a 2.80 x 1022 atom sample of the element is 2.09 g. Name the element and give its chemical symbol Name the element and give its chemical symbol. Find moles using NA: 2.80 x 1022 atoms x 1 mol atoms = .0465 mol 6.02 x 1023 atoms M = mass/mol = 2.09 g/.0465 mol= 44.9 g/mol 44 From periodic table – mass number corresponds to Sc – periodic table number corresponds to Sc scandium. Example: The uranium used in nuclear fuel exists naturally in several minerals Calculate how many grams of uranium are several minerals. Calculate how many grams of uranium are found in 100.0 grams of carnotite, K2(UO2)2(VO4)2•3H2O. Using molecular mass in calculations: 1.) Determine the mass (g) of 4.03 x 1024 molecules of stearic acid, C18H36O2. 2.) The hemoglobin content of blood is about 15.5 g/100 mL blood. The molar mass of hemoglobin is about 64 The molar mass of hemoglobin is about 64,500 g/mol & there are 4 g/mol there are Fe atoms in a hemoglobin molecule. Approximately how many Fe atoms are present in the 6.0 L of blood in a typical adult? Chemical reactions follow the law of conservation of mass. Balanced chemical equations Chemical equations should be balanced to follow the law of conservation of mass. Total mass of each element on the reactant side must equal the total mass of reactant side must equal the total mass of each element on the product side. Total charge of reactant side must equal charge of reactant side must equal the total charge of product side. Combustion reactions –studio 3 Reactions that occur between oxygen (O2) and another element in a compound. When the other compound is a hydrocarbon, the products of complete combustion are carbon dioxide and water vapor carbon dioxide and water vapor. Hydrocarbons are molecular compounds composed of only hydrogen and carbon composed of only hydrogen and carbon and are a class of organic compounds. Practice balancing eqns: C3H8 (g) + O2 (g) C5H10 (g) + O2 (g) CO2 (g) + H2O(l) CO2 (g) + H2O(l) St Stoichiometric calculations Calculating the mass of a product from the mass of a reactant requires: A balanced chemical reaction Molar mass of the reactant mass of the reactant Molar mass of the product Problem: How much carbon dioxide would be formed if 10.0 grams of C5H12 were completely burned in oxygen? C5H12 (g) + O2 (g) ---> CO2 (g) + H2O(l) Calculating the Mass Percent Composition of a Compound Problem: What is the mass percent of carbon and What is the mass percent of carbon and fluorine in halothane, C2HBrClF3? Determining the Empirical and Molecular Formulas of Determining the Empirical and Molecular Formulas of a Compound from Its Mass Percent Composition 1.) Dibutyl succinate is an insect repellent used against household ants and roaches. Its composition is 62.58% C, 9.63% H and 27.79% O. Its experimentally determined molecular mass is 230 g/mol. What are the empirical and molecular formulas of dibutyl succinate? A schematic diagram of the combustion device used to analyze substances for carbon and hydrogen (sulfur & nitrogen also w/ modifications). Empirical formula from combustion analysis Example: Ibuprofen is a compound used in painkillers. When a 2.174 g sample is burned completely in oxygen, it yields 6.029 g CO2 & 1.709 g H2O as the only products. What is the empirical formula of ibuprofen? Mass Spectrometry and Mass Spectrometry and Molecular Mass All Mass spectrometers (MS) separate atoms and molecules by first converting them into ions and then separating those ions based on the ratio of their masses to their electric charges. Mass spectrometers are instruments Mass spectrometers are instruments used to determine the mass of substances substances. Mass Spectrometer Mass Spectrometer Mass Spectra Determining the Molecular Determining the Molecular Formula The molecular formula can be determined from the percent th composition and mass spectral data. Example A combustion analysis of an unknown combustion analysis of an unknown compound indicated that it is 92. 23% C and 7.82% H. The mass spectrum and 7.82% H. The mass spectrum indicated the molar mass is 78 g/mol. What is the molecular formula of this What is the molecular formula of this unknown compound? Chemical Equations and Stoichiometry Chemical Equations and Stoichiometry Stoichiometry includes all the Stoichiometry includes all the quantitative relationships involving: atomic and formula masses chemical formulas. formulas • Mole ratio from the balanced chemical equation is central conversion factor equation is a central conversion factor. Example: Example: If 454 g of NH4NO3 decomposes, how how much much N2O and H2O are formed? 1.) Write the balanced equation. 2.) Convert reactant mass to moles of reactant. 3.) Convert moles of reactant to moles of Convert moles of reactant to moles of product using coefficients in equation. 4.) Convert moles of product to mass of Convert moles of product to mass of product. This answer is called “theoretical yield”. Percent Yield Yi Theoretical Theoretical Yield: the calculated amount of product formed amount of product formed Actual Actual Yield: the measured amount of product formed product formed Percent Yield = Yield Actual Yield Actual Yield x 100% 100% Theoretical Yield Percent Yield example Yi Aluminum burns in bromine liquid producing aluminum bromide. In a certain experiment, 6.00 g of aluminum was reacted with an excess of bromine to yield 50.3 g aluminum bromide. Calculate the theoretical and percent yields for this experiment. Reactions involving limiting li reagents In In a given reaction, there is not enough of one reagent to use up the other reagent one th completely. completely. The reagent in short supply LIMITS the The reagent in short supply LIMITS the quantity quantity of product that can be formed. LIMITING LIMITING REACTANTS Reactants Products 2 NO (g) + O2 (g) NO (g) Limiting Limiting reactant = Excess reactant Excess reactant = 2 NO2(g) NO Example: Example: Mix 5.40 g of Al with 8.10 g of Cl2. What mass of Al2Cl6 can form? 1.) Write a balanced chemical equation. 2.) Determine the limiting reagent – limits Determine the limiting reagent limits the amount of product. 3.) Use the limiting reagent to calculate the Use the limiting reagent to calculate the theoretical product yield. 4.) If asked, calculate the amount of If th reagent in “excess” that is left after the reaction is complete. ti Consecutive Reactions, Simultaneous Reactions & Overall Reactions Multistep synthesis is often unavoidable. Reactions carried out in sequence are called consecutive reactions. When substances react independently and at the same time the reaction is a simultaneous reaction. Overall Reactions and Intermediates The Overall Reaction is chemical The Overall Reaction is a chemical equation that expresses all the reactions occurring in single overall reactions occurring in a single overall equation. An intermediate is a substance produced in one step and consumed in produced in one step and consumed in another during a multistep synthesis. Example: Carbon dioxide produced in the combustion of ethane is absorbed by a barium hydroxide solution to produce 0.506 g barium carbonate (solid). How many grams of ethane gas must have been burned? Assume 100% yield have been burned? Assume 100% yield. (I) C2H6 (g) + O2 (g) CO2 (g) + H2O (l) BaCO3 (aq) + H2O (l) (II) CO2 (g) + Ba(OH)2 (aq) 1.) Balance reactions. 2.) From mass of barium carbonate, calculate moles of From mass of barium carbonate calculate moles of carbon dioxide. 3.) From moles of CO2, calculate mass of ethane burned. ...
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