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Atoms and molecules are so tiny that
they need to be counted in very large quantities in
order to result in a “workable” number.
The mole
The mole (abbreviation: mol) is used to count particles. 1 mole of
anything is 6.02 ´ 1023 of that thing.
Molar mass
We cannot count out a mole of particles in the lab, therefore a
connection between moles and mass is needed. The molar mass is the
mass (in grams) for 1 mole of the particle. Molar mass (also often
called Molecular mass, formula weight, etc.) is found by adding the
atomic masses (multiplied by the number of that atom) for each atom
in the molecule. Molar mass can then be used in dimensional
analysis conversions as the equality between grams and moles. The
molar mass value always goes with the “grams” in such conversions.
Percent Composition
“Percent” is part / whole ´ 100. The “part” is the mass of an
individual element while the “whole” is the mass of the whole
molecule or sample. These calculations can be done with lab
data—grams of both the individual element and the whole sample—or
with chemical formulas. In the case of chemical formulas, the molar
mass is used in the calculations.
Empirical Formula
The empirical formula is the lowest possible ratio of atoms in a
molecule. The ratio of atoms is the same as the ratio of moles of
atoms. The mass of each element is converted to moles and then all
the moles are divided by the smallest to reach the lowest possible
whole number ratio. That ratio is used as the subscripts in writing
the empirical formula.
Molecular Formula
The molecular formula is the actual ratio of atoms in a molecule.
The molar mass of the empirical formula is compared to the given
molar mass of the molecular formula. The comparison results in a
factor that is used to multiply the subscripts of the empirical
formula to reach the subscripts of the molecular formula.
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