May/Caiafa
Calculation of Molecular
Formula
May/Caiafa’06
This calculation uses the molecular mass and
the empirical formula for a compound.
1. Determine the empirical mass.
2. Divide the molecular mass by the empirical mass. After this
division you will have a number that:
• Is the ratio of the molecular to empirical formula; for example if
the # is 2 then the molecular formula would be double the
empirical formula.
• Will be multiplied by the subscripts of the empirical formula to
get the true or molecular formula
3. Multiply all of the subscripts by the whole number from step 2.
May/Caiafa
Calculation of Molecular
Formula
A compound has an empirical formula of C4H4O
and a molar mass of 136 grams per mole.
What is the molecular formula.
1. C4H4O = 4(12.01) + 4(1.008) + 16.00 =
68.07
2. 136/68.07 =
2
3. C4x2 H4x2 O1x2 = C8 H8 O2
Empirical
Mass
Ratio of molecular
to empirical
Molecular Formula