Chapter 3: ex. P2O5 molecular mass = 2(30.97 amu) + 5(16.00 amu) = amu

Transcription

1 Molecular Mass and Formula Mass for molecular compounds: the molecular mass is the mass (in amu) of one molecule of the compound molecular mass = atomic masses of elements present Chapter 3: ex. P2O5 molecular mass = 2(30.97 amu) + 5(16.00 amu) = amu Calculations with Chemical Formulas and Equations for ionic compounds: the formula mass is the mass (in amu) of one formula unit of the compound formula mass = atomic masses of elements present ex. Ca(ClO3)2 formula mass = amu + 2(35.45 amu) + 6(16.00 amu) = amu The Mole Using NA as a Conversion Factor SI base unit for amount of substance the mole is a counting unit 1 mole contains the same number of particles as there are atoms in exactly 12 g of 12C by definition: 1mol = x particles Avogadro s number, NA = x 1023 mol!1 so: 1 mol Li = x 1023 Li atoms NA relates amount of substance in mol to number of particles* * particles may be atoms, molecules, formula units or ions depending on sample under consideration examples: Calculate the amount (in mol) of molybdenum in a sample containing x 1025 atoms Mo. 1 mol H2O = x 1023 H2O molecules 1 mol NaNO3 = x 1023 NaNO3 formula units Calculate the number of SO3 molecules in 1.25 mol SO3.

2 Molar Mass What Does 1 Mole Look Like? Molar Mass of an Element - mass (in g) per mole of an element. units g/mol Molar Mass of a Compound - mass (in g) per mole of a compound. units g/mol molar mass compound = molar mass of elements present Molar Mass of an Element Molar Mass of an Element calculate the molar mass of an element: ex: Determine the mass (in g) of mol Sn. convert average atomic mass (in amu/atom) to molar mass (in g/mol) ex: Determine the molar mass of rhodium. use molar mass as a conversion factor relating sample mass (in g) to amount of substance (in mol) combine molar mass and NA as conversion factors... ex: Determine the number of atoms in g Sn. ex: Determine the mass (in g) of a single atom of Sn.

3 Molar Mass of a Compound Molar Mass of a Compound - mass (in g) per mole of a compound. units g/mol molar mass compound = molar mass of elements present ex: Calculate the molar mass of phosphorus pentachloride. ex: Calculate the number of molecules in 10.0 mg PCl5. ex: Calculate the mass (in g) of a single PCl5 molecule. Compound Stoichiometry the subscripts in a chemical formula tell us the ratio of how elements combine use these ratios as conversion factors in calculations ex: Consider calcium nitrate. Write the chemical formula. What relationships exist between... mass in g mass in amu amount in mol # of formula units # of calcium ions # of nitrate ions mol of calcium ion mol of nitrate ion # s of Ca, N, and O atom mol of Ca, N, and O Compound Stoichiometry Calculations Consider a 25.0 g sample of Ca(NO3)2. Determine the following: a. molar mass b. mol Ca(NO3)2 c. mol Ca 2+ d. mol NO3! e. mass (in g) of O f. number of NO3! ions g. number of formula units h. number of O atoms i. mass (in g) of 1 formula unit Mass Percent Composition composition of compound with respect to percent mass of each element present consider AxBy: x(molar mass A) mass% A = x 100 molar mass AxBy y(molar mass B) mass% B = x 100 molar mass AxBy and: mass % A + mass % B = 100% ex: Determine the mass % composition of barium nitrate.

4 Chemical Formulas: Empirical Formula vs. Molecular Formula empirical formula: relative numbers of atoms present subscripts in chemical formula are smallest possible whole number ratio molecular formula: exact numbers of atoms present subscripts in chemical formula may or may not be smallest possible whole number ratio Determination of Empirical and Molecular Formulas: Using Mass Percent Composition Data ex: Vitamin C (molar mass = g/mol) is composed of 40.9% C, 4.57% H, and 54.5% O by mass. Determine the empirical and molecular formula of Vitamin C. 1. Determine the mol of each element present. assume a 100 g sample and use mass % composition to interpret g of each element present convert g element! mol element 2. Determine the mol ratio of elements present in the smallest whole numbers. Determination of Empirical and Molecular Formulas: Using Mass Percent Composition Data (Cont d) ex: Vitamin C (molar mass = g/mol) is composed of 40.9% C, 4.57% H, and 54.5% O by mass. Determine the empirical and molecular formula of Vitamin C. 3. Divide the actual molar mass of the compound by the molar mass of the empirical formula. This will equal a small whole number, n. 4. Multiply the subscripts of the empirical formula by n! molecular formula

5 Another Formula Determination Example: Stoichiometry of Chemical Reactions A mg sample of a compound composed of only cadmium and phosphorus is analyzed and found to contain mg Cd. Answer questions like: Determine the empirical and molecular formulas of this compound. The actual molar mass of the compound is g/mol. Consider the reaction: CaO (s) + 3 C (s) " CaC2 (s) + CO (g) g/mol g/mol g/mol g/mol Calculate the mass (in g) of C required to react completely with g CaO. How many g of reactant B are required to react completely with x g of reactant A? How many g of product C can be formed from the complete reaction of x g of reactant A? Reaction Yields theoretical yield: maximum amount of product that can form in a reaction; typically reported in g or mol Calculate the mass (in g) of CaC2 that can form from the complete reaction of g CaO. 1 actual or experimental yield: actual amount of product collected from a reaction percent yield: percentage of the theoretical yield that is actually collected actual yield % yield = " 100 theoretical yield

6 Identification of the Limiting Reactant The limiting reactant is the reactant that limits the theoretical yield of product. it is the reactant that is completely consumed during the course of the chemical reaction as soon as one of the reactants is gone, the reaction must stop! TO CALCULATE A THEORETICAL YIELD, YOU MUST ALWAYS START WITH THE LIMITING REACTANT!! Identification of the Limiting Reactant: (a not very scientific example) to build a perfect s more you need: 2 graham crackers 2 marshmallows 1 square of chocolate Consider 2 sets of ingredients: A 50 graham crackers 50 marshmallows 25 squares chocolate B 60 graham crackers 75 marshmallows 22 squares chocolate Identification of the Limiting Reactant: (a not very scientific example) set up s more reaction tables: 2 graham crackers + 2 marshmallows + 1 square chocolate " 1 s more before change after graham crackers + 2 marshmallows + 1 square chocolate " 1 s more before change after Identification of Limiting Reactant: Calculations and Reaction Tables 100 g CaC2 and 100 g H2O are combined and allowed to react. identify the limiting reactant calculate the theoretical yield of products determine the mass of the excess reactant that remains unconsumed when the reaction is complete complete the reaction table

7 Calculations to fill in the reaction table: determine mol of each reactant 1 mol CaC2 100 g CaC2 " = 1.56 mol CaC g 1 mol H2O 100 g H2O " = 5.55 mol H2O g Calculations to fill in the reaction table: pick one reactant; calculate mol of 2 nd reactant required to react completely with the 1 st reactant 2 mol H2O 1.56 mol CaC2 " = 3.12 mol H2O 1 mol CaC2 or conversely 1 mol CaC mol H2O " = 2.78 mol CaC2 2 mol H2O Calculations to fill in the reaction table: compare mol of 2 nd reactant required for complete reaction to mol present in the system here - more H2O is present (5.55 mol) than is required (3.12 mol) to completely consume CaC2 H2O is the excess reactant OR there is less CaC2 present (1.56 mol) than is required (2.78 mol) to completely consume H2O present CaC2 is the limiting reactant before 1.56 mol 5.55 mol 0 0 change!1.56 mol!3.12 mol mol mol after mol 1.56 mol 1.56 mol before 1.56 mol 5.55 mol 0 0 change!1.56 mol!(2 1)1.56 mol +(1 1)1.56 mol +(1 1)1.56 mol after mol 1.56 mol 1.56 mol

8 Calculations based on the reaction table: determine the theoretical yield of each product g 1.56 mol Ca(OH)2 " = 116 g Ca(OH)2 1 mol Ca(OH) g 1.56 mol C2H2 " = 40.6 g C2H2 1 mol C2H2 determine the mass of excess reactant unconsumed when the reaction is complete Homework Problem: TiO2 (s) + 4 C (s) + 6 Cl2 (g)! 3 TiCl4 (g) + 2 CO2 (g) + 2 CO (g) 4.15 g 5.67 g 6.78 g g/mol g/mol g/mol 3 TiO2 + 4 C + 6 Cl2! 3 TiCl4 + 2 CO2 + 2 CO before mol mol mol change after g 2.43 mol H2O " = 43.8 g H2O 1 mol H2O A Final Reaction Stoichiometry Example: Gravimetric Analysis A g sample containing some unknown amount of potassium superoxide (KO2) and other substances is treated with excess H2O; the following reaction occurs * : 4 KO2 (s) + 2 H2O (l) " 4 KOH (s) + 3 O2 (g) *note: of the substances present in the original sample, only the KO2 will react with water Assuming that the you achieve complete conversion of KO2 to KOH, determine the mass percent KO2 in the original sample if the experiment yields g KOH.