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Formula Weights. The weight of an individual molecule or formula unit Also known as molecular mass or molecular weight Is calculated by adding the masses of the atoms in a single molecule or formula unit Unit is amu Mass of 1 molecule of H 2 O = 2(1.0 amu ) + 1(16.0 amu) = 18.0 amu
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Formula Weights • The weight of an individual molecule or formula unit • Also known as molecular mass or molecular weight • Is calculated by adding the masses of the atoms in a single molecule or formula unit • Unit is amu • Mass of 1 molecule of H2O = 2(1.0 amu) + 1(16.0 amu) = 18.0 amu • Formula weight of NaCl = 1(23.0 amu) + 1(35.5 amu) = 58.5 amu
Molar Mass Molar mass is the mass of 1 mole of anything in grams. 1 mole C atoms = 6.022 x 1023 atoms = 12.01 g 1 C atom = 12.01 amu 1 mole C atoms = 12.01 g C Molar mass of C = 12.01 g 1 mole lithium atoms = 6.941 g of Li
Practice: Calculation of Formula Weight and Molar mass of a Compound • Calculate the formula weight of (NH4)2SO4. (Ans:132.1 amu) • Calculate the molar mass of Al(NO3)3. (Ans: 213.0 g/mol)
Using Moles • Moles provide a bridge from the mass of a substance in grams and the number of molecules or atoms (formula units) • Calculate the number of moles and the number of atoms in a 10.0 g sample of Al. Ans: 0.370 mol Al 2.23 x 1023 Al atoms
Practice: Interconvert among moles, grams and molecules (or atoms) 1. Calculate the mass in grams of 6.05 mol of NaHCO3. Strategy: mol NaHCO3 g NaHCO3 (Ans: 508 g NaHCO3) 2. How many molecules of C2H6O are in 75.0 g of C2H6O ? Strategy: g C2H6O Mole C2H6O molecules of C2H6O (9.81x1023molecules C2H6O)