Section 3 : Chemical Quantities and Stoichiometry 3-1 → area of study devoted to the quantities of materials consumed and produced in a reaction. @ Atomic Masses → in 1961 a system was devised to assign atomic masses to each element → to do this, a "standard" was needed. → an isotope of carbon, carbon-12 ($^{12}$C), was used as the standard and was assigned a value of exactly 12 amu → amu = atomic mass units So, why is carbon's atomic mass 12.01 amu on the Periodic Table? → Answer: most elements occur as a mixture of isotopes and natural carbon is found as 98.89% $^{12}$C and 1.11% $^{13}$C atoms → So, (.9889)(12 amu) + (.0111)(13 amu) = 12.01 amu → atomic mass is a "weighted avg." of isotopes this is why atomic masses of elements are expressed as decimals on the Periodic Table → note that not a single carbon atom weighs 12.01 amu!! This is an average weight. Ex: lithium (Li) ↔ 6.941 amu

Chemical Quantities and Stoichiometry Lecture Notes

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