Chemistry Chapter 4 Student Notes Lesson 2 Molar Mass (pg. 131 – 136) A “MOLE” = 6.02 x 1023 of anything; atoms, candy bars, beans, etc. ATOMIC MASS Because of the very small “actual” mass of a single atom, the mass of elements is defined in the following way: Atomic Mass = Mass of 1 mole of atoms of that element Therefore, when chemists speak of the “mass” of an element we are usually referring to the mass of one mole (6.02 x 1023) of atoms. For Carbon the mass of one mole is 12 grams: a.k.a. Molar mass of Carbon Carbon = 12.0 g/mole (as shown on the periodic table) Said another way, the gram atomic mass of carbon = 12 g/mole IMPORTANT FACT The MASS # on the PERIODIC TABLE is the average Molar Mass for each individual element MOLECULAR MASS Because of the very small “actual” mass of a single molecule, the mass of a molecular compound is defined in the following way: Molecular Mass = Mass of 1 mole of the molecules When chemists speak of the “mass” of a molecular compound we add the mass of a moles’ worth (6.02 x 1023 atoms) of the total number of each atom in the molecule. For sucrose (table sugar) we have the molecular formula C12H22O11 and the mass is 342 grams: a.k.a. Molar mass of sucrose Carbon = + Hydrogen = + Oxygen = = 12 atoms x 12.0 g/mole Carbon contributes 144g 22 atoms x 1.01 g/mole Hydrogen adds 22g 11 atoms x 16.0 g/mole Oxygen adds 176g 342 g/mole Most accurately, we would say that the gram molecular mass of sucrose = 342 g/mol (i.e., a mole of sucrose molecules has a mass of 342.3 g) Chemistry Chapter 4 Student Notes – cont’d FORMULA MASS Because of the very small “actual” mass of a single formula unit of an ionic compound, the mass of ionic compounds is defined in the following way: Formula Mass = Mass of 1 mole of the formula unit When chemists speak of the “mass” of an ionic compound we add the mass of a moles’ worth (6.02 x 1023 atoms) of the total number of each atom in the formula unit of the compound. For sodium chloride (table salt) we have the formula NaCl and the mass would be 58.5 grams: a.k.a. Molar mass of sodium chloride Sodium = + Chloride = = 1 atom x 23.0 g/mole 1 atom x 35.5 g/mole Sodium contributes 23g Chloride adds 35.5g 58.5 g/mole Most accurately, we would say that the gram formula mass of sodium chloride = 58.5 g/mol (i.e., a mole of Sodium Chloride has a mass of 58.5 g) Chemistry Chapter 4 Student Notes – cont’d Problems: Gram Atomic Mass What is the Mass (in grams)of: Helium Atoms (He) (atomic mass of Helium = 4.00 g/mole) 1 mole: 2 moles: 8 moles: Iron Atoms (Fe) (atomic mass of Iron = 55.8 g/mole) 1 mole: 2 moles: 8 moles: Problems: Gram Molecular Mass What is the Mass (in grams)of: Hydrogen Molecules (H2) note: the periodic table shows the atomic mass of a single Hydrogen atom = 1.01 g/mole 1 mole: 2 moles: 8 moles: Oxygen Molecules (O2) note: the periodic table shows the atomic mass of a single Oxygen atom = 16.0 g/mole 1 mole: 2 moles: 8 moles: Sugar (sucrose) Molecules (C12H22O12) (the atomic mass of H and O are listed above, Carbon = 12.0 g/mole) 1 mole: 2 moles: 8 moles: Chemistry Chapter 4 Student Notes – cont’d Problems: Gram Formula Mass What is the Mass (in grams) of : One mole of KMnO4 K = _______ g/mole + Mn = ________ g/mole + O = ________ g/mole x 4 Oxygens in KMnO4 = ________ g/mole adding them all together = ________ g per formula unit One mole of KCl One mole of Na2SO4 Two moles of Sodium Chloride Four moles of Iron II Oxide Chemistry Chapter 4 Student Notes – cont’d Problems: Gram Atomic Mass Helium Atoms (He) (atomic mass of Helium = 4.00 g/mole) 1 mol 4g =4g 1 mol 2 mol 4g =8g 1 mol 8 mol 4g = 32 g 1 mol Iron Atoms (Fe) (atomic mass of Iron = 55.8 g/mole) 1 mol 55.8 g = 55.8 g 1 mol 2 mol 55.8 g = 112 g 1 mol 8 mol 55.8 g = 446 g 1 mol Problem: Gram Molecular Mass What is the Mass in grams of this many moles of… Hydrogen Molecules (H2) (atomic mass of Hydrogen = 1.01 g/mole) 1 mole H2 2 mol H 1 mol H2 1.01 g = 2.02 g mol H 2 mole H2 2 mol H 1 mol H2 1.01 g = 4.04 g mol H 8 mole H2 2 mol H 1 mol H2 1.01 g = 16.2g mol H Chemistry Chapter 4 Student Notes – cont’d Oxygen Molecules (O2) (atomic mass of Oxygen = 16.0 g/mole) 1 mole O2 2 mol O 1 mol O2 16.0 g = 32.0 g mol O 2 mole O2 2 mol O 1 mol O2 16.0 g = 64.0 g mol O 8 mole O2 2 mol O 1 mol O2 16.0 g = 256 g mol O Sugar (sucrose) Molecules (C12H22O12) (atomic mass of H and O listed above, Carbon = 12.0 amu) 12 C’s x 12 g/mol = 144 g/mol 22 H’s x 1.01 g/mol = 22 g/mol 12 O’s x 16.0 g/mol = 192 g/mol = 358 g/mol of C12H22O12 1 mole: 358 g 2 moles: 716 g 8 moles: 2,860 g Chemistry Chapter 4 Student Notes – cont’d Problem: Gram Formula Mass What is the Mass (in grams) of : One mole of KMnO4 K= + Mn = + O = 16.0 g/mole x 4 Oxygen’s in KMnO4 = adding them all together = 39.1 54.9 64.0 158.0 One mole of KCl K = 39.1 g/mole + Cl = 35.5 g/mole = 74.6 g One mole of Na2SO4 Formula = Na2SO4 2 x 23.0 two Na’s 46 + 32.1 one S + 4 x 16.0 four O’s + 32 + 64 Two moles of Sodium Chloride Formula = NaCl 23.0 + 35.5 = 58.5 g/mole NaCl 2 mol 58.5 g = 117 g NaCl 1 mol Four moles of Iron II Oxide Formula = FeO 55.8 + 16.0 = 71.8 g/mole FeO 4 mol 71.8 g = 287 g FeO 1 mol = 142 g/mole Na2SO4 g/mole g/mole g/mole g/mole