Molecular vs Empirical Formula Chemistry Name Block Read the information below and then answer the questions that follow Definitions: Molecular formula the total number of atoms of each element in a compound Empirical formula the simplest/smallest ratio of each element in a compound Examples: Substance Molecular Formula Simplifying the Ratio Empirical Formula Sugar C 6 H 12 O 6 Water Hydrogen Peroxide Salt (Sodium Chloride) H 2 O H 2 O 2 NaCl Ribose C 5 H 10 O 5 Molecular & Empirical Formulas Part 1 Calculate the Molar Masses and % composition for both the empirical and molecular formula below Then, determine a similarity and difference between the two Substance Molecular Formula (MF) Empirical Formula (EF) Sugar C 6 H 12 O 6 CH 2 O Molar Mass %Composition % C: % H: % O: % C: % H: % O: Similarity between MF & EF Difference between MF & EF
Molecular & Empirical Formulas Part 2 Substance Molecular Formula Empirical Formula Water H 2 O H 2 O Salt (Sodium Chloride) NaCl NaCl You should have noticed that for both substances, the Molecular & Empirical Formulas are the same Explain why it is possible for both the Molecular & Empirical Formulas to be the same Molecular & Empirical Formulas Part 3 Substance Molecular Formula Empirical Formula Glucose C 6 H 12 O 6 CH 2 O Ribose C 5 H 10 O 5 CH 2 O You should have noticed that both substances have the same empirical formula Explain why it is possible for two different MFs and have the same EF
Molecular & Empirical Formulas Part 4 Substance Molecular Formula (MF) Empirical Formula (EF) Hydrogen Peroxide H 2 O 2 = Molar Mass 3402 g/mol 1701 g/mol The MF subscripts has a common divisible number, 2 Therefore, each subscript was divided by 2 in order to obtain the EF Calculate the ratio of the molar masses: = = What do you notice about the ratio of the molar mass and common divisible number? Critical Thinking: If you knew the empirical formula and molar mass of your molecular formula, how would you try to find the Molecular formula?
Practice: 1 Complete the table below by writing the empirical formula for each molecular formula Molecular Formula Empirical Formula P 4 O 10 N 2 O 5 C 12 H 27 C 20 H 25 O 10 2 A compound has an empirical formula of CH 2 and a molecular mass of 420g/mol Determine its molecular formula 3 The empirical formula was determined to be PBr 5 What is the molecular formula if the molar mass of the molecular formula was found to be 431 g/mol 4 A combustion analysis gives the following empirical formula of C2H4O Determine the molecular formula knowing that the molecular mass = 13216g/mol 5 Naphthalene is a carbon and hydrogen containing compound often used in moth balls The empirical formula is C 5 H 4 and its molar mass is 12816g/mol Find the molecular formula
Determining Empirical Formulas from Mass Data Ex: Assume you have a sample that is 0504 g hydrogen and 400 g oxygen 1 Convert grams moles for each element 2 Simplify the mole ratio for each element by dividing each number (of moles) by the smallest # of moles 3 Use the smallest whole-number mole ratio as the subscripts for the empirical formula Practice: 1 A 5325 g sample of methyl benzoate, a compound used in the manufacture of perfumes, is found to contain 3758 g of carbon, 0316 g of hydrogen and 1251 g of oxygen What is the empirical formula of methyl benzoate? 2 A 2498g sample of an unknown compound was analyzed and found to contain 135 g Ca, 108 g O, and 0675 g H What is the empirical formula of the compound?
Determining Empirical Formulas from Percent Composition Data: 2 Methods Ex: Determine the empirical formula for a compound containing 740% carbon (C), 865% hydrogen (H), and 173% nitrogen (N) by mass 1 Assume a 100 gram sample for the compound The percent composition of each element will then be equal to the mass of the element found in the 100 g sample Assume a 100 g sample: C: 740 % 74 g C in 100 g sample H: 865 % 865 g H in 100 g sample N: 173% 173 g N in 100 g sample 3 Simplify the mole ratio for each element by dividing each number (of moles) by the smallest # of moles C: H: = 501 5 = 696 7 2 Convert grams moles for each element C: 74 = H: 865 = N: 173 = 4 Use the smallest whole-number mole ratio as the subscripts for the empirical formula C: 5 H: 7 C5H7N N: = 100 N: 1 Note: If the simplified mole ratios are not all whole numbers, multiply by the smallest factor that will produce whole number subscripts Practice: 1 A compound is found to contain 6352% iron and 3648% sulfur Determine the empirical formula
2 Ascorbic Acid (vitamin C) is 4092% Carbon, 458% Hydrogen and 5450% Oxygen by mass Calculate the empirical formula of Ascorbic Acid ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Mixed Practice: 1 Determine the empirical formula of the given molecular formulas below Molecular Formula HgBr2 Empirical Formula Molecular Formula C8H14 Empirical Formula C2H6O C3H6 2 A compound has an empirical formula of CF3 and a molar mass of 13802 g/mol What is the molecular formula of the compound? 3 A compound has an empirical formula of CaCl2 Determine the molecular formula of the substance if its molar mass is 11098 g/mol
4 A 100 gram sample of a compound contains 722 grams of magnesium and 278 grams of nitrogen The molar mass of this compound is 20062 g/mol a What is the empirical formula of the compound? b What is the molecular formula? 5 Fat makes up a major portion of all soaps A fat used in many soaps is 765% carbon, 122% hydrogen, and 113% oxygen a What is its empirical formula? b What is the molecular formula of a fat with a molar mass of 705 g/mol?