Unformatted text preview: of H’s and O’s, more about this in Ch. 6) Application: Law of Constant Composition Chemical Formula We write a Chemical Formula to summarize the elements present in
the compound, e.g. H2O. Two Samples of Iron oxide are found, each decomposed to
Fe and O use elemental symbol for each atom in compound
use subscript to indicate number of atoms of each type.
( “1” is implied if nothing written, and use parentheses when needed.) Elements are written in this order: (metal left, non-metal right)
most Examples: NaCl not ClNa
CO2 not O2C
CHCl3 not HCl3C or Cl3CH, etc. (exception, OH-) Mass Fe (g) To
most 167.54 72.00 Sample 2 Mass O (g) Sample 1 125.65 48.00 Are they the same Iron Oxide? Look at the mass ratios:
amu amu mass Fe 167.53 g Fe
125.65 g Fe
= 2.327 ≠ 2.618 =
72.00 g O amu
48.00 g O
Analysis: based on 4 sig figs, ratios are different.
Conclusion: Not the same Iron Oxide!
(FYI: Hematite, Fe2O3 vs Magnetite, Fe3O4 , notice how chemical
formulas are often a more meaningful summary than masses). http://old.iupac.org/publications/books/princip...
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