# Molecular Mass: Solved Questions on Relative Molecular Mass

### What is Molecular Mass?

The amount or quantity of a compound (or molecule) in chemistry can be measured in a variety of ways. The measured quantities will include:

1. Mole of a substance.
2. Molecular Mass of a compound.
3. Molar Mass of a compound and so on.
Please read on molar mass, worked examples and it's differences with molecular mass here.

The term molecule refers to the composition and arrangement of atoms expressed in ratios. The atoms forming a molecule can be similar or different. For instance:

• O2 is oxygen molecule (where the oxygen atoms are combined in a ratio of 1:1). Similarly, ozone (O3) is also a molecule; but in this case, the oxygen atoms are combined in a ratio of 1:1:1.
• Water (H2O) is a molecule composed of hydrogen and oxygen in the ratio 2:1.

Recall that all elements have got their respective atomic masses. For instance, hydrogen (H) has an atomic mass of 1.00794 while that of carbon (C) is 12.011. The atomic mass of all 108 elements can be seen on the periodic table.

The molecular mass of a substance or molecule is usually defined in relation to 12C (Carbon-12). Based on this relationship with carbon-12, it’s preferably called the Relative Molecular Mass (RMM).

The relative molecular mass value is obtained from the sum of the atomic masses of the constituent atoms or elements that make up the molecule. Its unit is the Atomic Mass Unit (amu) or Dalton (Da).

In order to calculate the relative molecular mass, we will have to take into consideration the atomic masses of each atom present in the molecule.

### Solved Questions on Molecular Mass

#### Example 1

Calculate the relative molecular mass of methane (CH4). (Carbon=12.001 and H=1.00794).

• Molecular Mass = Atomic mass of carbon + Atomic mass of hydrogen
• Methane is composed of one atom of carbon and 4 atoms of hydrogen.
• 12.001 + (1.00794 x 4)

Molecular Mass of Methane = 16.033 amu

#### Example 2

Calculate the relative molecular mass of water (H2O). (Oxygen=15.9994 and Hydrogen=1.00794).

• Molecular Mass = Atomic mass of hydrogen + Atomic mass of oxygen
• Water contains 2 atoms of hydrogen and 1 atom of oxygen.
• (1.00794 x 2) + 15.9994

Molecular Mass of Water = 18.015 Da

#### Example 3

Calculate the relative molecular mass of ethanol (C2H5OH). (Carbon=12.001, Oxygen = 15.9994 and Hydrogen = 1.00794).

• Molecular Mass = Atomic mass of carbon + Atomic mass of hydrogen + Atomic mass of oxygen
• Ethanol contains 2 atoms of carbon, 6 atoms of hydrogen and 1 atom of oxygen.
• (12.001 x 2) + (1.00794 x 5) + 15.9994

Molecular Mass of Ethanol = 45.036 amu

Note: We used 3 significant digits in all our answers. This is so because the relative molecular mass is always defined and expressed in terms of 12C (Carbon-12) whose atomic mass has 3 significant digits.

The atomic mass of 12C is 12.011

A major challenge encountered when calculating molecular mass is that it becomes difficult or impossible to calculate especially when the relative molecular mass of large molecules, polymers and macromolecules are involved.

Examples of large molecules (with indefinite molecular masses) include carbohydrates, cellulose and complex sugars.

The large molecules (above) have no specific chemical formula throughout their volume.

Relative Molecular Mass prove to be useful only when we calculate substances with small and definite molecular sizes. This had been proven through the modifications of Dalton's atomic theory.