Ch 3 Stoichiometry and Calculations.pdf

# Most elements in nature are mixtures of most elements

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Most elements in nature are mixtures of Most elements in nature are mixtures of more than 1 isotope “Average atomic mass” incorporates the “Average atomic mass” incorporates the relative abundance of each isotope in nature p+ p+ n n p+ n p+ n p+ p+ n n p+ p+ n n p+ n p+ n p+ p+ n n n p+ p+ n n p+ n p+ n p+ p+ n n n n 6 6 6 6 protons 6 neutrons 6 protons 7 neutrons 6 protons 8 neutrons 12 C 13 C 98.892 % 1.108 % Trace amount (<0.001%) 14 C (stable) (stable) (radioactive) 24 S. Ensign, stoichiometry and calculations

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Average atomic mass of C found in nature p+ p+ n n p+ n p+ n n p+ p+ n n p+ n p+ n n n p+ p+ n n p+ n p+ n n n n p n p+ p+ p n p+ p+ p n p+ p+ 6 protons 6 neutrons 6 protons 7 neutrons 6 protons 8 neutrons 12 C 13 C 14 C 98.892 % 12 amu 1.108 % 13.00335 amu Trace amount (<0.001%) 25 S. Ensign, stoichiometry and calculations Naturally occurring bromine is composed of two isotopes which have atomic masses of 78.918336 and 80.916289 amu Isotope 1 is present at an abundance of 50 69% amu. 50.69% while isotope 2 is present at an abundance of 49.31% . Use this information to determine the average atomic f t ll i b i mass of naturally occurring bromine 26 S. Ensign, stoichiometry and calculations The atomic weights provided in a Periodic Table are the average atomic weights of the naturally occurring elements 27 S. Ensign, stoichiometry and calculations Formula mass • Atoms have atomic masses (atomic Atoms have atomic masses (atomic weights) • Compounds have formula masses Compounds have formula masses (formula weights) T l l t f l th To calculate formula mass, sum the atomic masses of all atoms in the l l molecule 28 S. Ensign, stoichiometry and calculations