CHEM1011 Lecture Notes - Lecture 2: Unified Atomic Mass Unit, Molar Mass, Atomic Mass
15 Aug 2018
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Molar mass: h2 (2 g/mol); d2 (4 g/mol) Densities: h2o (1 g/ml); d2o (1. 1 g/ml) Molar mass: h2o (18 g/mol); d2o (20 g/mol) Atomic mass: average mass in amu of the atoms of naturally occurring mixture of isotopes. Atomic mass unit: 1/12 th of mass of a carbon-12 atom. Example: calculate the average atomic mass of naturally occurring magnesium. Atomic mass = 23. 98504 x 78. 99% + 24. 98589 x 10. 00% +25. 98259 x 11. 01 % *average atomic mass= sum of (fraction each isotope) x (mass each isotope) Copper occurs naturally as a mixture of two isotopes: 63cu (abundance 69. 09%) and 65cu (30. 91%). Their atomic masses are 62. 930 amu and 64. 928 amu, respectively. Whenever dealing with percentages, a useful trick is to consider 100 of whatever the items are. In this case, consider you have 10000 atoms of natural copper (2 decimal places) Of these, 6909 atoms will be 63cu, of weight = 6909 x 62. 930 amu.
