Defined simply, molecular weight is the mass of all the atoms present in a substance. It is also referred to as the molecular mass of a substance. It refers to the total distribution of protons and neutrons that compose the nucleus. Take for example water. Water is a molecule composed of hydrogen and oxygen. Molecules are stable groups of two or more atoms, which are electrically neutral. Oxygen and hydrogen are atoms present in water. The distribution of atoms is two atoms of hydrogen and one atom of oxygen. To determine water’s molecular weight, you need to add the weight of all the three atoms that are present in one water molecule.
Importance of Molecular Weight
The science of getting the molecular mass of a substance is actually important. Why? This is because determining molecular weight is the initial step used by researchers to analyze unknown substances. Once the molecular weight of a molecule is known, there is the possibility to begin the assembly of structural information about the molecule. It can be done together with obtaining additional information about the molecule, like its basic atomic constituents.
Determining the Molecular Weight
The molecular weight of materials that are solid or liquid at room temperature can be determined by following one of the colligative properties of solutions:
- Freezing-point lowering. This is a good method for knowing the molecular weights of smaller organic molecules.
- Osmotic pressure. This is used for determining the molecular weights of larger organic molecules.
- Boiling-point elevation. This method is used less frequently.
In choosing which technique needs to be used, it depends upon the number of particles in the solution and not on the nature of the particle. The size is the most of important of the determinants. It is required that the molecule’s size should be small enough to dissolve in the solution but should be large enough not to join in the phase change.
On the other hand, determining the gas weight is different from getting the solid and liquid weight. A gaseous substance's molecular weight is determined by Avogadro’s Law. The Law of Avogadro states that a gas, under given temperature and pressure conditions, contains a specific number of molecules. To this effect, the weight of equal volumes of different gases under the same conditions of temperature and pressure is equivalent to the weights of molecules of the gases.
The molecular mass of gel permeation, hemoglobin, albumin, amino acids, sodium, and polymers are commonly determined together with protein weight and liquid weight. Polymer and protein weight is regarded as containing millions of organic molecules. So the molecular weight of every substance is determined in different ways and yet gathered for the same purpose. In the very essence of molecular weight, the mass of hemoglobin, albumin, protein, amino acid, sodium, water, or any other substance is determined to start new studies on substances.
Molecular weight doesn't have to be determined for every substance. If you need to know the molecular weight of a common substance, reference books and online calculators can give you an aid in that.