2 Friday, April 15th: “A” Day Monday, April 18th: “B” Day Agenda Homework questions/problems?Quiz over section 7.2Begin 7.3: “Formulas & Percentage Composition”In-Class Assignments:Practice pg. 243: #1-4Practice pg. 245: #1-3
3 Quiz 7.2: “Relative Atomic Mass and Chemical Formulas” You can use your book and your guided notes for this walk-talk quiz…Remember: answer only the question that corresponds to the month you were born in.If you were born in November, answer question #2. If you were born in December, answer question # 7.Once everyone has answered their question, get up, walk around, and talk/compare answers with others.
4 7.3: “Formulas and Percentage Composition” The percentage composition is the percentage by mass of each element in a compound.Percentage composition helps verifya substance’s identity.Percentage composition can also be used to compare the ratio of masses contributed by the elements in two different substances.
6 Empirical FormulaAn actual formula shows the actual ratio of elements or ions in a single unit of a compound.Empirical formula: a chemical formula that shows the simplest ratio for the relative numbers and kinds of atoms in a compound.For example, the empirical formula for hydrogen peroxide is HO, while the actual formula is H2O2
7 Determining Empirical Formulas You can use the percentage composition for a compound to determine its empirical formula.Convert the percentage of each element in the compound to grams.Convert from grams to moles using the molar mass of each element as a conversion factor.Compare these amounts in moles to find the simplest whole-number ratio among the elements.
8 Determining Empirical Formulas To find the simplest whole-number ratio, divide each amount in moles by the smallest of all the amounts of moles.This will give a subscript of 1 for the atoms present in the smallest amount.Finally, you may need to multiply all of the amounts of moles by a number to convert all subscripts to small, whole numbers.The final numbers you get are the subscripts in the empirical formula.
9 Determining an Empirical Formula form Percentage Composition (Sample Problem G, pg. 242) Chemical analysis of a liquid shows that it is 60.0% C, 13.4% H, and 26.6% O by mass. Calculate the empirical formula of this substance.1. Assume that you have a 100 g sample so that each percentage is the same as the amount in grams:for C: % = 60.0 g Cfor H: 13.4% = 13.4 g Hfor O: 26.6% = 26.6 g O
10 Sample Problem G, continued… 2. Use the molar mass to convert each amount in grams to amount in moles:
11 Sample Problem G, continued… Divide each number of moles found by the smallest number of moles found. (1.66 moles O)Carbon: mol = mol C1.66 molHydrogen: mol= mol HOxygen: mol = 1 mol OThese numbers are within experimental error to be considered whole numbers so the empirical formula is: C3H8O
12 Additional PracticeFind the empirical formula given the following composition:26.58% K, % Cr, and % OAssume 100 g sample:26.58 g K35.35 g Cr38.07 g O
13 Additional Practice Use molar mass to convert from grams to moles. 26.58 g K X 1 mole K = mol K39.10 g K35.35 g Cr X 1 mole Cr = mol Cr52.00 g Cr38.07 g O X 1 mole O = mol O16.00 g O
14 Additional PracticeDivide each number of moles found by the smallest number of moles found (.6798 mol)..6798 mol K = 1 mol K.6798 mol.6798 mol Cr = 1 mol Cr2.379 mol O = 3.5 mol O
15 Additional Practice K2Cr2O7 Since these are not whole numbers, multiply each one by 2 to get whole numbers.1 mol K (2) = mol K1 mol Cr (2) = mol Cr3.5 mol O (2) = 7 mol OThese ARE whole numbers, so the empirical formula is:K2Cr2O7
16 Molecular Formulas are Multiples of Empirical Formulas The formula for an ionic compound shows the simplest whole-number ratio of the large numbers of ions in a crystal of the compound.A molecular formula is a whole-number multiple of the empirical formula.The molar mass of any compound is equal to the molar mass of the empirical formula times a whole number, n.n (empirical formula) = molecular formula
17 Comparing Empirical and Molecular Formulas CompoundEmpirical FormulaMolecular FormulaFormaldehydeCH2OAcetic AcidC2H4O22X the empirical formulan = 2GlucoseC6H12O66X the empirical formulan = 6
18 Determining a Molecular Formula from an Empirical Formula (Sample Problem H, pg. 245) The empirical formula for a compound is P2O5. Its experimental molar mass is 284 g/mol. Determine the molecular formula of the compound.1. Use the periodic table to find the molar mass of the empirical formula:For P: 2(30.97) = g/molFor O: 5(16.00) = g/molMolar mass of P2O5 = g/mol
19 Sample Problem H, continued… Find the multiplier, n:n = experimental molar mass of compoundmolar mass of empirical formulan = 284 g/mol = Hint: the bigger #g/mol always goes on top!3. To find the molecular formula, simply multiply the empirical formula by 2:2 (P2O5) = P4O10
20 Additional Practice Determine the molecular formula for the following: Molar mass: g/molEmpirical formula: OCNClFind molar mass of empirical formula:O = g/molC = g/molN = g/molCl = 35.5 g/mol = g/mol
21 Additional Practice 2. Find the multiplier, n: n = experimental molar mass of compoundmolar mass of empirical formulan = g/mol = 377.52 g/mol3. To find the molecular formula, simply multiply the empirical formula by 3:3 (OCNCl) = O3C3N3Cl3
22 In-Class Assignments You Must SHOW WORK! Practice pg. 243: #1-4Practice pg. 245: #1-3We will finish this section next time…