CHAPTER 8: CHEMICAL COMPOSITION

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1 CHAPTER 8: CHEMICAL COMPOSITION Active Learning: 1-4, 6-8, 12, 18-25; End-of-Chapter Problems: 3-4, 9-82, 84-85, 87-92, , , 111, 113, 119, ATOMIC MASSES: COUNTING ATOMS BY WEIGHING Atoms are too small to weigh directly e.g. one carbon atom has a mass of g too inconvenient an amount to use! need more convenient unit for an atom s mass atomic mass unit (amu) Carbon-12 was chosen as the reference and given a mass value of 12 amu 1 amu = 1/12 th the mass of a carbon-12 atom Masses for all other elements are measured relative to mass of a carbon-12 atom Weighted Average Atomic Mass of an Element Why is carbon s mass on Periodic Table amu, NOT amu?! Atomic masses reported on the Periodic Table are weighted averages of the masses of all the naturally occurring isotopes for each element based on their percent natural abundance i.e., the percentage of existing atoms that are a specific naturally occurring isotopes. Ex. 1 If % of carbon exists as carbon-12, which has a mass of , while 1.108% exists as carbon-13, which has a mass of , calculate the average atomic mass for carbon. average atomic mass = ( )( amu) + ( )( amu) Ex. 2 The atomic masses of the three naturally occurring isotopes or argon, Ar-36 (0.3365%), Ar-38 (0.0632%) and Ar-40 ( %), are amu, amu, and amu, respectively. Calculate the average atomic mass for argon. page 1 of 10

2 8.3 THE MOLE Atomic Masses and Molar Masses If 98.93% of C atoms are carbon-12 and 1.07% of C atoms are carbon-13, then the (weighted average) atomic mass of carbon is calculated as follows: ( amu)(0.9893) + ( amu)(0.0107) = amu So how many carbon atoms are present in grams of carbon? This number was determined experimentally to be It was named Avogadro s number, to honor Italian scientist Amedeo Avogadro who first proposed in 1811 that the volume of a gas depended only on the number (not type) of atoms or molecules present at a given temperature and pressure AVOGADRO'S NUMBER (N A )= (4 sig figs) How big is this? If hydrogen atoms were laid side by side, the total length would encircle the Earth about a million times. The mass of Olympic shotput balls is about equal to the mass of the Earth. The volume of softballs is about equal to the volume of the Earth. page 2 of 10

3 THE MOLE CONCEPT 1 mole (abbreviated mol) = items Similar to: 1 dozen = 12 items 1 dozen doughnuts = 12 doughnuts 1 mole of doughnuts = doughnuts MOLE CALCULATIONS I a. How many eggs are in 3 dozen eggs? b. How many eggs are in 3.00 moles of eggs? c. How many moles of C atoms are present in a sample of C atoms? 8.5 MOLAR MASS Thus, the mass of 1 mole of C atoms is g (or g/mol) 1 mole ( ) is the amount of atoms of any element that has a mass in grams equal to the mass of ONE atom in amu. The atomic masses reported for each element in the Periodic Table give the average atomic mass in amu and the molar mass in g/mol. Molar mass (MM): Mass in grams of 1 mole of any element or compound Example: Use the Periodic Table to provide the molar mass of each element below: a. Al: b. Si: c. Ar: d. Sn: page 3 of 10

4 Molar mass (MM): Mass in grams of 1 mole of any element or compound To obtain the molar mass of a compound, multiply the molar mass of each element by the number of each present, then add them all up. Example: Determine the molar mass of each of the following compounds: a. O 2 : 2 (molar mass of O) = 2 (16.00 g/mol) = g/mol b. NaCl: c. CO 2 : d. H 2 SO 4 : e. Ca 3 (PO 4 ) 2 : Do not worry about rounding molar mass to the correct number of sig figs! Molar mass will rarely be the measurement that limits the number of sig figs for a calculation. CONVERSIONS AMONG MASS, NUMBER OF MOLES, AND NUMBER OF PARTICLES Use the dimensional/unit analysis method: 1. Write the units of the final answer. 2. Write the given information related to the answer. 3. Determine the unit factors (Avogadro s #, molar masses) necessary to solve the problem using the given information Ex. 1: How many moles of helium are in 5.00 g of helium? page 4 of 10

5 Ex. 2: How many moles of neon are in 25.0 g of neon? Ex. 3: How many neon atoms are in 25.0 g of neon? Ex. 4: How many moles of H 2 O are in 50.0 g of H 2 O? Ex. 5: How many H 2 O molecules are in 50.0 g of H 2 O? Ex. 6: How many H atoms are in 50.0 g of H 2 O? page 5 of 10

6 MOLAR VOLUME: The volume occupied by 1 mole of any gas Avogadro's Law: At the same temperature and pressure, equal volumes of gases contain the same number of molecules. Standard temperature and pressure (STP): T=0 C and P=1.00 atm At STP, 1 mole of gas occupies 22.4 L! 3 sig figs Write 2 unit factors using this information: Mole Calculations with Molar Volume: Ex. 1 What mass of CO 2 occupies a volume of 25.0 L at STP? Ex. 2 What is the volume occupied by 10.0 g of propane gas (C 3 H 8 ) at STP? Ex. 3 How many He atoms are present in a 10.0 L balloon filled with He gas at STP? page 6 of 10

7 8.6 PERCENT COMPOSITION OF COMPOUNDS Mass Percent Composition (or Mass Percent): The mass of each element in a compound divided by the mass of the entire compound. Mass of Element Mass Percent of Element = 100% Total Mass of Compound Steps to determine percentage composition: 1. Calculate the total mass of each individual element in the compound 2. Add up all the masses of each element to get the total mass of the compound 3. Divide the mass of each individual element with the total mass of compound Ex. 1 What is the percent composition by mass of each element in H 2 O? Ex. 2 What is the percent composition by mass of each element in K 2 SO 4? % H % O % K % S % O Ex. 3 What is the percent composition by mass of each element in Al 2 (CO 3 ) 3? % Al % C % O page 7 of 10

8 Ex. 4: Calculate the mass of iron in 50.0 g of rust, iron(iii) oxide. Ex. 5: Calculate the mass of silver present in 125 g of silver carbonate. Ex. 6 What mass of nitrogen is present in 75.0 g of ammonium nitrate? 8.7 FORMULAS OF COMPOUNDS Empirical Formula: Simplest whole-number ratio of atoms in a compound where the term empirical means derived from experiment Molecular Formula: Fill in the table below: Chemical formula of a compound that expresses the actual number of atoms present in one molecule. The molecular formula will either be exactly the same or some multiple of the empirical formula! glucose, C 6 H 12 O 6 caffeine, C 6 H 10 N 4 O 2 acrylonitrile, C 3 H 3 N Empirical Formula Molecular Formula page 8 of 10

9 8.8 CALCULATION OF EMPIRICAL FORMULAS Guidelines for Determining the Empirical Formula of a Compound 1. Find the # of moles of each element in the compound. 2. Divide each # of moles of each element by smallest # of moles to get ratio of atoms. 3. Get a whole number ratio for all atoms in the compound: If within 0.1 of a whole number, round to that whole number If any ratio ends close to 0.5 multiply ALL subscripts by 2 If any ratio ends close to 0.33 or 0.66 multiply ALL subscripts by 3 Ex. 1: Determine the empirical formula for nickel oxide if a sample of nickel oxide consists of g of nickel and 7.26 g of oxygen? empirical formula: name: Ex. 2: Ascorbic acid, known more commonly as Vitamin C, is a hydrocarbon derivative, a compound that consists of carbon, hydrogen, and oxygen. If a g sample of ascorbic acid contains g of carbon and 1.14 g of hydrogen, determine the empirical formula for ascorbic acid. Empirical Formula of ascorbic acid: page 9 of 10

10 8.9 CALCULATION OF MOLECULAR FORMULAS Determining the Molecular Formula of a Compound 1. For the Molecular Formula, you will be given the molar mass of the compound, and you need to calculate the molar mass of the empirical formula. 2. Divide the molar mass of the compound by the molar mass of the empirical formula to get the factor by which to multiply each subscript in the empirical formula. Ex. 3: If the ascorbic acid in Ex. 2 above has a molar mass of g/mol, determine its molecular formula. Empirical and Molecular Formulas from Percent Composition: 1. Assume g of the compound is present, so change percent units to grams. 2. Follow the same steps for determining empirical formula and molecular formula. Ex. 4: Quinine is used as an antimalarial drug. An analysis of quinine indicates the compound consists of 74.03% carbon, 7.47% hydrogen, 8.64% nitrogen, and 9.86% oxygen. If quinine's molar mass is 325 g/mol, determine the empirical and molecular formulas for quinine. Empirical Formula of Quinine: Molecular Formula of Quinine: page 10 of 10

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