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The Molecular Formula of a Compound

    The molecular formula, also known as the actual formula, describes the number of atoms of each element that make up a molecule.

Molecular formula subscripts = n x Empirical formula subscripts. (n = 1, 2, 3...)

The empirical formula of ribose (sugar) is CH2O. In a separate experiment, using a mass spectrometer, the molar mass of ribose was determined to be 150 g/mol. What is the molecular formula of ribose?

Step 1
Find the molar mass.


C -> 1 x 12.0 = 12.0
H -> 2 x 1.01 = 2.02       = Total of 30.02 g.
O -> 1 x 16.0 =  16.00

Step 2
Divide the molar mass by the molar mass that you've found.

150 g/mol                          
------------------ = 5 (rounded)
30.02 g/mol                          

Step 3
Multiply the answer you obtain from Step 2, with the compound formula given.

Molecular Formula = (CH20)x5 = C5H10O5

C5H10O5 is your final answer, and is also the molecular formula of Ribose.