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To determine the molecular formula, the molecular weight of the substance must be known. There are several methods for determining molecular weight:

Vapor Density Method: In this method, a known volume of gas is weighed at a known temperature. After converting the volume to STP, we can determine what fraction of a mole that volume represents. It is then elementary to determine the molecular weight of the substance.

Here is an example.
Cryoscopic Method: In this method, the freezing point depression of a solvent is measured after a known quantity of the unknown substance is added to it. Several basic calculations will yield the substances molecular weight.

Here is an example.
Vapor Pressure Osmometry: In this method, the change in the vapor pressure of a solvent is measured after a known amount of a substance is dissolved in it.

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Mass Spectrometry: In mass spectrometry, a sample of the substance in question is vaporized and bombarded by high energy electrons. This results in the formation of molecular fragments whose masses can be measured directly. Formation of a molecular ion peak yields the molecular weight of the substance.
Neutralization Equivalent: If a substance is sufficiently acidic (most carboxylic acids and some phenols), it may be titrated by a standard solution of sodium hydroxide. The neutralization equivalent is identical to the molecular weight of the substance.

Here is an example.

Chemical Formulas

Molecular Mass Determination