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12/11/2013 The Mole, Molar Mass and Avogadro’s Number Mass Relationships in Chemical Reactions Look at the “atomic masses” on the periodic table. What do these represent? Atomic mass is the mass of the atom in atomic mass units (amu). One amu is defined as a mass equal to one-twelfth the mass of one carbon-12 atom Carbon-12 is the carbon isotope that has 6 protons and 6 neutrons 1 12/11/2013 Carbon’s atomic mass on the periodic table is 12.01 amu not 12.00 amu. Why the difference? 12.01 amu represents the average mass of the naturally occurring mixture of isotopes of carbon Two problems 1. Atomic masses do not convert easily to grams 2. They can’t be weighed (they are too small) The Mole A mole is defined as the # of atoms of carbon-12 in exactly 12 grams of carbon-12 The mole is a counting unit. It is used to compare the # of atoms, molecules, ions, in any substance There are 6.02 x 1023 particles in one mole of any substance. This number is called Avogadro’s number (NA) 2 12/11/2013 1 dozen = 12 items 1 gross = 144 items 1 mole = 6.02 x 1023 items One mole of carbon-12 has a mass of exactly 12 grams or 12 g/mol. This is defined as the molar mass, the mass of one mole of units. e.g. 1 mol Li = 6.94 g Li is expressed as 6.94g/mol The molar mass, molecular weight, or formula weight equations used to determine the mass in grams of one mole of a substance 3 12/11/2013 The molar mass of calcium phosphate, Ca3(PO4)2 = (3 x 40.08 g/mol Ca) + (4 x 2 x 16.0 g/mol O) + (2 x 31.0 g/mol P) = 310.2 g/mol Ca3(PO4)2 Calculate molar masses (to 2 decimal places) CaCl2 110.984 g/mol (NH4)2CO3 98.088 g/mol Pb3(PO4)2 811.546 g/mol C6H12O6 180.162 g/mol Avogadro’s, NA, relationship is used to convert from the number of moles of a substance to the number of atoms, ions, or molecules of a substance and vice versa (unit = mol-1) These values may be used to determine the number of atoms in a given mass of an element MOLAR MASS 4 12/11/2013 or the mass of one atom, ion, or molecule The subscripts within a chemical formula also indicate the number of moles of that element or ion in one mole of that compound. (ratio = mol/mol) These ratios are useful when determining the number of moles or particles in a particular compound. MOLE to MOLE RATIO 5 12/11/2013 How many atoms of oxygen are in 3.5 grams of calcium phosphate? Once the number of moles of the substance is known, you can determine the number of molecules or formula units of the substance. Molecules are found in covalent compounds. Ionic compounds do not have molecules their smallest subunits are called formula units. How many formula units of calcium phosphate are there in 0.211 moles of Ca3(PO4)2 ? 6 12/11/2013 PRACTICE Calculate the number of moles in each of the following: 64.1 g of aluminum 850.5 g of ammonium phosphate Calculate the mass of each of the following; 0.275 mol mercury 9.37x10-3 mol calcium chlorate Calculate the amount in moles in each of the following; 8.08 x 1022 atoms of krypton 2.997 x 1025 atoms of oxygen in vanadium(V) oxide Calculate the mass of the following: 3.01 x 1021 atoms of cobalt 1,20 x 1025 atoms of helium Calculate the number of atoms in each of the following: 0.697 g of gallium 69.45 g of lanthanum Determine the number of molecules in 0.0500 mol of hexane, C6H14 7