CHEMICAL REACTIONS: THE MOLE AND FORMULAS. Honors Chemistry Unit 9: Chemical Reactions
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1 CHEMICAL REACTIONS: THE MOLE AND FORMULAS Honors Chemistry Unit 9: Chemical Reactions
2 OBJECTIVES: Define the mole and Avogadro s Number Define molar mass Determine molar mass for elements and compounds using the periodic table Convert molar mass of compounds to moles Calculate percent composition of atoms in compounds Determine empirical formula for compounds Determine molecular formula for compounds
3 The MOLE The MOLE is a unit of measure, much like one dozen or one pair One mole is equal to 6.02 x particles 1 dozen eggs = 12 eggs 1 pair of shoes = 2 shoes 1 mole = 6.02 x particles Particles = compounds, molecules, or atoms Avogadro s Number is 6.02 x Avogadro got this number from the number of atoms in exactly 12g of Carbon-12
4 Using the MOLE in Chem The mole is used as a mass comparison for any element in the periodic table. We can compare the mass in grams of any two elements only if they have the same number of moles. it s like comparing the mass of 1 dozen marbles to 1 dozen eggs...
5 Atomic Mass / Molar Mass Atomic mass = mass of one atom of any element in atomic mass units (amu s) - found using the average weighted atomic mass on the periodic table! Molar mass = mass in grams of one mole of atoms or molecules How they are related: Atomic mass of OXYGEN is 16 amu 1 mole of OXYGEN has a mass of 16 grams, SO Molar mass of OXYGEN is 16 grams/mol
6 Try it: What is the molar mass of: Cu N Fr
7 The terms Molar mass is the generic term Also seen as: Gram molecular mass (for molecules/covalent compounds) Gram formula mass (for ionic compounds) Gram atomic mass (for elements) We will use MOLAR MASS to describe all of these!
8 Molar Mass Example For compounds: What is the molar mass of MgCO 3?
9 Using Molar Mass: Percent Composition Percent Composition = percent by mass of atoms in a compound Indicates the relative amount of each element present in a compound To determine the percent composition of compounds, you need to convert using molar mass Percent by mass can be calculated using: Mass of element X 100 = percent by mass Mass of compound
10 Steps for Percent Composition Step 1: Calculate molar mass Step 2 : Divide the subtotal for each element s mass by the molar mass. Step 3: Multiply by 100 to convert to a percentage.
11 Example: % Composition 1. Calculate the molar mass of water, H 2 O. Step 1 Step 2 H- 2(1.01 g)= 2.02 g O 1(16.0 g)= 16.0 g g/mole 2.02/18.02 X 100 = 11.21% 16.0 /18.02 X 100 = % Water is composed of % hydrogen and % oxygen.
12 You try it: Calculate the percentage composition of sucrose: C 12 H 22 O 11
13 Empirical Formula Empirical formula = the formula with the smallest whole number mole ra2o of the elements May or may not be the same as the molecular formula If the two formulas are different the molecular formula will always be a simple mul2ple of the empirical formula The empirical formula for hydrogen peroxide is HO The molecular formula for hydrogen peroxide is H 2 O 2
14 Determining Empirical Formula If the % composi2on is given you can assume the total mass of the compound is 100 grams and that the percent by mass of each element is equal to the mass of that element in grams. The mass of each element can be converted to moles by dividing the molar mass of the element. EXAMPLE: The percent composi2on of a sulfur oxide is % S and % O g S = mol S g/mol S g O = mol O g/mol O
15 Empirical Formula Check it! NOTE: The calculated mole values may not always be whole numbers In these cases all the mole values must be multiplied by the smallest factor that will make them whole numbers
16 Empirical Formula STEPS % to Mass Mass to Mole Divide by smallest Mul2ply un2l whole
17 Molecular Formulas Two or more substances with distinctly different properties can have the same percent composition and the same empirical formula The subscripts in an empirical formula indicate the simplest whole-number ratio of moles of the elements in a compound The simplest ratio does not always indicate the actual number of moles in the compound
18 Determining Molecular Formulas A molecular formula specifies the actual number of atoms of each element in one molecule or formula unit of the substance. The molar mass must be determined through experimenta2on and compared with the mass represented by the empirical formula.
19 Calcula2ng Molecular Formula Experimentally determined molar mass of the compound = n Mass of empirical formula of the compound A molecular formula can be represented as the empirical formula mul2plied by an integer n. molecular formula = (empirical formula)n The integer is the factor by which the subscripts in the empirical formula must be mul2plied to obtain the molecular formula
20 EXAMPLE: Molecular Formula
21 Try it: Analysis of a chemical used in photographic developing fluid indicates a chemical composition of 65.45% C, 5.45%H, and 29.09% O. The molar mass is found to be g/mol. Determine the molecular formula.
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