Final answer:
To determine the molecular mass using an empirical formula, you must know the empirical formula mass and the molar mass of the compound. Divide the molecular mass by the empirical formula mass to find the number of empirical formula units per molecule, which can be used to determine the molecular formula.
Step-by-step explanation:
Determining Molecular Mass from the Empirical Formula
To use your empirical formula to determine the molecular mass of a compound, you need to know a few important pieces of information. First, you must calculate the empirical formula mass (EFM), which is the sum of the average atomic masses of all the atoms represented in an empirical formula. Once you have the EFM, you will need the molar mass of the compound, which can often be determined using a mass spectrometer.
With both the empirical formula mass and the molecular (or molar) mass, you can then calculate the number of empirical formula units per molecule, designated as n. This is done by dividing the molecular mass by the empirical formula mass. The result, which should be a whole number or very close to it, indicates the multiplier needed to convert the empirical formula into the molecular formula.
For example, if the empirical formula of a compound is CH₂O and its molar mass is determined to be approximately 120 g/mol, and the empirical formula mass is 30 g/mol, then dividing the molar mass by the empirical formula mass gives us 4, which means the molecular formula would be C₄H₈O₄, or simply (CH₂O)₄.