Organic molecules

Table of Contents

1. Organic molecules
2. Hydrocarbons
   1. Names and structures of hydrocarbons
3. Reactions of hydrocarbons
   1. Substitution reactions
   2. Addition reactions
4. Functional groups
   1. Identifying functional groups
5. Alcohols
   1. Structure of alcohols
   2. Physical properties of alcohols
6. Reactions of alcohols
   1. Oxidation reactions
   2. Carboxylic acids
   3. Alcohols, carboxylic acids and esters
   4. Identification of alcohols
7. Nitrogen in organic molecules
   1. Amines
   2. Amino acids
8. Determination of formulae and structures
   1. Empirical formulae
   2. Molecular mass and formula

1. Additional resources:
   1. Glossary
   2. Print content

Molecular mass and formula

In the previous example, it was stated that the molecular formula of the compound was the same as the empirical formula. This is of course not always the case, although the molecular formula will always be a whole number multiple of the empirical formula.

This section reminds you how to calculate the molecular formula of an unknown compound once you have worked out the empirical formula.

Calculation of molecular mass and formula

You are normally given enough information to work out the number of moles of a compound in a certain amount (mass) of the compound.

You can then work out the molecular mass (M) using the relationship:

M = =

Sometimes you will have to do a multi-stage calculation in order to determine the mass and number of moles present. (See example below.)

Once the molar mass has been found and the empirical formula is known, it is easy to work out the molecular formula. Follow the steps shown below.

Divide the molar mass by the relative mass of the 'empirical formula unit'.

This should give an answer close to a whole number.

Multiply the empirical formula unit by this whole number.

Follow this example to help understand the processes involved.

Example

An unknown compound was calculated to have an empirical formula of CH_3O. It was found that when a 2.00 g sample of the compound was vaporised at 200 °C at 1.00 atm pressure the gas had a volume of 1.25 L.

Calculate the molecular formula of this compound.

Click to reveal the steps required to solve this problem.

Calculate number of moles (n)

Calculate the molar mass (M)

Compare the molar mass to the empirical formula

Complete the empirical formulae and structure problems .

The answers can be accessed here .