Presentation on theme: "Chapter 6 Chemical Composition 2006, Prentice Hall."— Presentation transcript:
1 Chapter 6Chemical Composition2006, Prentice Hall
2 CHAPTER OUTLINE The Mole Concept Molecular & Molar Mass Calculations Using the MolePercent CompositionChemical FormulasCalculating Empirical FormulasMolecular Formulas
3 Why is Knowledge of Composition Important? everything in nature is either chemically or physically combined with other substancesto know the amount of a material in a sample, you need to know what fraction of the sample it isSome Applications:the amount of sodium in sodium chloride for dietthe amount of iron in iron ore for steel productionthe amount of hydrogen in water for hydrogen fuelthe amount of chlorine in freon to estimate ozone depletionTro's Introductory Chemistry, Chapter 6
4 THE MOLE CONCEPTChemists find it more convenient to use mass relationships in the laboratory, while chemical reactions depend on the number of atoms present.In order to relate the mass and number of atoms, chemists use the SI unit mole (abbreviated mol).
5 Avogadro’s number (NA) THE MOLE CONCEPTThe number of particles in a mole is called Avogadro’s number and is 6.02x1023.1 mole6.02 x 1023equals toAvogadro’s number (NA)
6 A mole is a very large quantity THE MOLE CONCEPTA mole is a very large quantity6.02x1023If 10,000 people started to count Avogadro’s number and counted at the rate of 100 numbers per minute each minute of the day, it would take over 1 trillion years to count the total number.
7 THE MOLE CONCEPT 1 mole H atoms = 6.02x1023 H atoms 1 mole H2 molecules= 6.02x1023 H2 molecules= 2 x (6.02x1023) H atoms1 mole H2O molecules= 6.02x1023 H2O molecules= 2 x (6.02x1023) H atoms= 6.02x1023 O atoms1 mole Na+ ions= 6.02x1023 Na+ ions
8 Mass of 1 mol H atoms = 1.008 grams THE MOLE CONCEPTThe atomic mass of one atom expressed in amu is numerically the same as the mass of 1 mole of atoms of the element expressed in grams.Mass of 1 Mg atom = amuMass of 1 H atom = amuMass of 1 Cl atom = amuMass of 1 mol Cl atoms = gMass of 1 mol Mg atoms = gMass of 1 mol H atoms = grams
9 MOLE DAYChemists and chemistry students celebrate two days in the year in honor of the Mole and call them Mole Days.October 23rd6:02 a.m.Jun 2nd10:23 a.m.
10 Mass of one molecule of H2O MOLECULAR MASSThe sum of atomic masses of all the atoms in one molecule of a substance is called molecular mass, and is measured in amu.Mass of one molecule of H2O2 H atoms =2 (1.008 amu) = amu1 O atom =1 (16.00 amu) = amuMolecular mass18.02 amu
11 Relationship Between Moles and Mass The mass of one mole of atoms is called the molar massThe molar mass of an element, in grams, is numerically equal to the element’s atomic mass, in amu
12 MOLAR MASSThe mass of one mole of a substance is called molar mass, and is measured in grams.Mass of one mole of H2O2 mol H atoms =2 (1.008 g) = g1 mol O atom =1 (16.00 g) = gMolar mass18.02 g
13 CALCULATIONS USING THE MOLE Conversions between mass, mole and particles can be done using molar mass and Avogadro’s number.Avogadro’s numberMolar massMass of a substanceMoles of a substanceParticles of a substanceMMNA
14 Example 1: What is the mass of 5.00 mol of water? 18.02 90.1 1 3 significant figuresMolar mass
15 Example 2: Avogadro’s number Molar mass How many Mg atoms are present in 5.00 g of Mg?mass mol atoms16.02 x 102324.311.24x1023 atoms MgAvogadro’s numberMolar mass3 significant figures
16 Example 3: How many molecules of HCl are present in 25.0 g of HCl? mass mol molecules16.02 x 102336.4514.13 x 1023 molecules HCl3 significant figures
17 PERCENT COMPOSITIONThe percent composition of a compound is the mass percent of each element in the compound.Total mass of elementTotal mass of compound
18 Example 1:Calculate the percent composition of sodium chloride (NaCl).Step 1:determine molar mass of NaCl1 mol Na atoms =1 (22.99 g) = g1 (35.45 g) = g1 mol Cl atom =Molar mass58.44 g/mol
19 Example 1: Step 2: calculate the mass % of each element Sum = 100% % Na =% Cl =
20 Example 2: mass of sample = 1.63 g of zinc combines with 0.40 g of oxygen to form zinc oxide. Determine the % composition of the compound formed.Step 1:determine total mass of samplemass of sample =1.63 g g = 2.03 g
21 Example 2: Step 2: calculate the mass % of each element Sum = 100% % Zn =% O =
22 Example 3:Calculate the percent composition of sodium hydroxide (NaOH).Step 1:determine molar mass of NaOH1 mol Na atoms =1 (23.0 g) = 23.0 g1 (16.0 g) = 16.0 g1 mol O atom =1 (1.01 g) = 1.01 g1 mol H atoms =Molar mass40.0 g/mol
23 Example 3:Calculate the percent composition of sodium hydroxide (NaOH).Step 1:determine molar mass of NaOH1 mol Na atoms =1 (23.0 g) = 23.0 g1 (16.0 g) = 16.0 g1 mol O atom =1 (1.01 g) = 1.01 g1 mol H atoms =Molar mass40.0 g/mol
24 CHEMICAL FORMULAS Molecular formula Empirical formula Shows the actual number of atoms in a compoundCan be written for molecular compounds onlyShows the simplest ratio of atoms in a compoundCan be written for molecular and ionic compounds
25 CHEMICAL FORMULAS Molecular Empirical Multiplier H2O2 H2O C6H12O6 C6H6
26 CHEMICAL FORMULASSeveral compounds may possess the same percent composition and empirical formula, but different molecular formulas.FormulaComposition% C % HMolar mass(g/mol)CH92.37.7C2H2C6H613.0226.04 (2x13.02)78.12 (6x13.02)
27 Finding an Empirical Formula convert the percentages to gramsskip if already gramsconvert grams to molesuse molar mass of each elementwrite a pseudoformula using moles as subscriptsdivide all by smallest number of molesmultiply all mole ratios by number to make all whole numbersif ratio ?.5, multiply all by 2; if ratio ?.33 or ?.67, multiply all by 3, etc.skip if already whole numbers
28 If, after dividing by the smallest number of moles, the subscripts are not whole numbers, multiply all the subscripts by a small whole number to arrive at whole-number subscripts.
29 CALCULATING EMPIRICAL FORMULAS Arsenic (As) reacts with oxygen (O) to form a compound that is 75.7% As and 24.3% oxygen by mass. What is the empirical formula for this compound?Assume 100 gStep 1:Percent to mass75.7% As75.7 g As24.3% O24.3 g O
30 CALCULATING EMPIRICAL FORMULAS Step 2:Mass to mole75.7 g As24.3 g OUse atomic mass of oxygen
31 CALCULATING EMPIRICAL FORMULAS Step 3:Divide by smallAs =O =
33 Example 1:Determine the empirical formula for a compound containing 11.2% H and 88.8% O.mol H =mol O =H2O
34 Example 2:Determine the empirical formula for a compound with the following percent composition: % C, 13.12% H, 34.73% O.C2H6Omol C =mol H =mol O =
35 Molecular formula = (empirical formula) n MOLECULAR FORMULASMolecular formula can be calculated from empirical formula if molar mass is known.Molecular formula = (empirical formula) n
36 Example 1:A compound of N and O with a molar mass of 92.0 g, has the empirical formula of NO2. What is its molecular formula?Mass of empirical formula =(16.0) = 46.0Molecular formula =2 x (NO2) = N2O4
37 Example 2:Calculate the empirical and molecular formulas of a compound that contains 80.0% C and 20.0% H, and has a molar mass of g.mol C =mol H =Empirical formulaCH3
38 Example 2: Empirical formula = CH3 Mass of empirical formula = (1.01) = 15.0Molecular formula =2 x (CH3) = C2H6
Your consent to our cookies if you continue to use this website.