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Formula Weights

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

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  1. 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

  2. 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

  3. 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)

  4. 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

  5. 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)

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