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# Atomic Mass and Molecular Mass ## Problem 1.2: Average Atomic Mass of Neon Calculate the Average Atomic Mass (Ne) using the following data: | Isotope | Atomic Mass (u) | Natural Abundance (%) | |---|---|---| | Ne | 19.9924 | 90.92 | | Ne | 20.9940 | 0.26 | | Ne | 21.9914 | 8.82...

# Atomic Mass and Molecular Mass ## Problem 1.2: Average Atomic Mass of Neon Calculate the Average Atomic Mass (Ne) using the following data: | Isotope | Atomic Mass (u) | Natural Abundance (%) | |---|---|---| | Ne | 19.9924 | 90.92 | | Ne | 20.9940 | 0.26 | | Ne | 21.9914 | 8.82 | **Solution** (19.9924u) (90.92) + (20.9940u) (0.26) + (21.9914u)(8.82) / 100 = 20.1707 u ## 1.7.3 Molecular Mass The molecular mass is the sum of the average atomic mass of all atoms in a molecule. It's calculated relative to the mass of one carbon-12 atom. **Example** The molecular mass of carbon dioxide (CO₂) is: 1(average atomic mass of C) + 2 (average atomic mass of O) = 1 (12.0 u) + 2(16.00) = 44.0 u **More Examples** * H₂O = 2x1.0 u + 16.0 u = 18 u * CH₃Cl = (6 x 12 u) + (5 x 1.0 u) + (35.5 u) = 112.5 u * H₂SO₄ = (2 x 1 u) + (32 u) + (4 x 16 u) = 98 u ## Problem 1.3: Mass of 1 Oxygen Molecule Find the mass of 1 molecule of oxygen (O₂) in amu (u) and grams. **Solution:** * Molecular mass of O₂ = 2 x 16 u = 32 u * Mass of 1 molecule = 32 u * Mass of 1 molecule of O₂ = 32.0 x 1.66056 x 10⁻²⁴ g = 53.1379 x 10⁻²⁴ g ## 1.7.4 Formula Mass Some substances do not contain discrete molecules. In these compounds, such as sodium chloride, cationic (sodium) and anionic (chloride) entities are arranged in a three-dimensional structure. NaCl is the formula that represents sodium chloride, though it's not a molecule. The formula mass is the sum of the atomic masses of the atoms present in the formula. ## Problem 1.4: Formula Mass Find the formula mass of: 1. NaCl 2. Cu(NO₂)₂ **Solution** 1. **NaCl:** * Average atomic mass of Na + average atomic mass of Cl = 23.0 u + 35.5 u = 58.5 u 2. **Cu(NO₂)₂:** * Average atomic mass of Cu + 2 x (average atomic mass of nitrogen + average atomic mass of three oxygen): (63.5) + 2(14+3 x 16) = 187.5 u ## Mole: Expressing Large Quantities A mole represents a large number of atoms, ions, or molecules. One mole is the amount of substance that contains as many particles as there are atoms in (or 0.012 kg) of carbon-12. **Definition** One mole is the amount of substance that contains as many elementary entities as there are atoms in 0.012 kg of carbon-12. **Number of atoms in 12 g of carbon-12:** 1.992648 x 10⁻²³ g/atom x 12 g/mol = 6.0221367 x 10²³ atoms/mol **Therefore, one mole is the amount of substance that contains 6.0221367 x 10²³ entities.** **Remember:** The number 6.0221367 x 10²³ is Avogadro's constant, "Nₐ". **Example:** * 1 mole of oxygen atoms = 6.022 x 10²³ atoms of oxygen * 1 mole of water molecules = 6.022 x 10²³ molecules of water * 1 mole of sodium chloride = 6.022 x 10²³ formula units of NaCl