Molecular formulae

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Terminology

Atomic mass: The sum of the number of protons and neutrons in one atom of an element

The relative formula mass of a molecule is similar to the atomic mass of an atom.

The relative formula mass can be found by finding the sum of the atomic masses of all of the atoms in the molecule.

The relative formula mass may be represented by \(M_r\)

Empirical formula: Simplest whole number ratio of atoms or ions of each element in a substance

Molecular formula: The actual number of atoms of each element in a molecule

Ethene, shown below, has a molecular formula of \(C_2H_4\) but an empirical formula of \(CH_2\) because there are twice as many hydrogen atoms as carbon atoms in each molecule of ethene.

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Examples

Question: What is the atomic mass of hydrogen?

Answer: 1

Explanation: Hydrogen contains one proton and no neutrons, so its atomic mass is \(1+0=1\)

Question: What is the atomic mass of oxygen?

Answer: 16

Explanation: Oxygen contains eight protons and eight neutrons. Therefore its atomic mass is \(8+8=16\)

Question: What is the relative formula mass of hydrogen?

Answer: 2

Explanation: Hydrogen exists as a diatomic molecule, \(H_2\). That means that its molecular mass is the mass of two hydrogen atoms, i.e.

\(1+1=2\)

Question: What is the empirical formula of glucose, \(C_6H_{12}O_6\)?

Answer: \(CH_2O\)

Explanation: There are equal numbers of oxygen and carbon atoms. There are twice as many hydrogen as carbon atoms.

Notice that all of the subscripts are divisible by 6; if you divide through by the highest common factor, you will get the empirical formula.

Question: What is the molecular formula of a molecule that has a relative formula mass of 58 and an empirical formula of \(C_2H_5\)?

Answer: \(C_4H_{10}\)

Explanation: The molecular formula of a molecule with the formula \(C_2H_5\) would be 29. This is half of 58. Therefore, if you multiply all of the subscripts by two, this will give you the desired relative formula mass.