I. Calculating Molar
Mass
- multiply atomic mass of each element by number of
atoms of that element in the formula (shown by the subscript)
- find the sum of all the atomic masses --this is
formula mass (unit is a.m.u.)
- express formula mass in grams (unit is g/mol). This
is the Molar Mass.
II. Calculating %
Composition (from formula)
- calculate formula mass
- divide the total atomic mass of each element by the
formula mass and multiply by 100
III. Calculating %
Composition (from masses of each element)
- divide the mass of each element by the total mass
of the compound and multiply by 100
IV. Calculating Empirical
Formula (from % Composition)
- convert % of each element to grams based on 100
grams of the compound
- multiply grams of each element by 1/molar mass that
element
- compare ratio of moles of each element and divide
each by the smallest
- if result in step 3 gives a ratio with decimal
equivalent to 1/4, 1/3, 1/2, 2.3, 3/4 instead of whole numbers, convert
to the fraction and multiply all ratios by the denominator or the
fraction
V. Calculating Empirical
Formula (from experimentally determined masses)
- multiply the mass of each element (in grams) by
1/molar mass of that element
- continue with steps 3 & 4 from IV above.
Example
VI. Finding Molecular
Formulas (when molar mass is known)
- calculate the empirical formula
- use the equation : (empirical formula mass) x =
molar mass
- find value for x: x = molar
mass/empirical formula mass
- multiply each subscript in empirical formula by
value for x
Example