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Atoms of different elements combine in a definite ratio to form molecules that form the smallest unit of a substance, into which a substance can be divided without changing its chemical properties.
The mass of a mole of a particular substance is given by the molecular mass of the substance, whereas the mass of a molecule of a substance is given by the molecular weight of the substance.
A few questions arise: “What is a mole of a substance ?”, “Is there any difference between average molecular weight and molecular weight ?”.
The physical and chemical properties of a substance are described to an extent by the average molecular weight and molecular mass of the substance.
Molecular weight
The Sum of atomic weight values for all atoms present in a molecule gives the molecular weight of the molecule. M.W or MW are the abbreviations used for molecular weight. Molecular weight is useful in analysing chemical reactions and is useful in determining the stoichiometry of chemical reactions.
Molecular weight is expressed in terms of Atomic mass unit(amu).
Calculation of Molecular Weight
The molecular weight of a compound is determined by its molecular formula.
- First of all, the atomic weight of each atom present in the molecule is determined.
- Secondly, the number of atoms of each type of element present in the molecule is determined.
- Then, the number of each type of atom is multiplied by its atomic weight.
- Lastly, all the products obtained in step 3 are added together to result in the molecular weight.
For Example, let’s calculate the molecular weight of butane.
The molecular formula of butane is C4H10.
-
- The atomic weight of carbon: 12.01
- The atomic weight of hydrogen: 1.01;
The molecular weight of butane = (Number of Carbon atoms
present in a molecule of butane)(Atomic weight of Carbon) + (Number of Hydrogen
atoms present in a molecule of butane)(Atomic weight of Hydrogen)
Molecular weight of butane = (4 ×12.01) + (10 ×1.01)
Molecular weight of butane = (48.04) + (10.1)
Molecular weight of butane = 58.14 amu
Average Molecular Weight
Many elements in nature have their isotopic forms. Isotopes of elements are two or more atoms that possess the same atomic number, that is they are present at the same position in the periodic table but they differ in nucleon members that is they have the same number of protons but a different number of neutrons. Due to the difference in the number of nucleon members isotopes possess different atomic weights.
In the calculation of average molecular weight, an average atomic weight of the element is considered rather than their normal atomic weight.
For example: In the calculation of the molecular weight of AlCl3, the average atomic weight of Chlorine is used which is 35.453.
The two stable isotopes of chlorine are 35Cl(75.77%) and 37Cl(24.23%). The average molecular weight of Chlorine is 35.453
Calculation of Average Molecular Weight
Just as the molecular weight of a compound, average molecular weight is determined by the molecular formula of the compound.
- First of all, the average atomic weight of each atom present in the molecule is determined.
- Secondly, the number of atoms of each type of element present in the molecule is determined.
- Then, the number of each type of atom is multiplied by its average atomic weight.
- Lastly, all the products obtained in step 3 are added together to result in the molecular weight.
Molecular Mass
The mass of a one-mole molecule of a compound gives the molecular mass of the molecule. One mole molecule of any substance is 6.022 × 1023 molecules. It is the mass of a single molecule of the compound with gram as the unit. As molecules are very tiny, therefore it is very difficult to individually calculate their masses thus relative molecular mass came into the picture and the isotope of Carbon is chosen as the standard atom for calculation of relative atomic masses and relative molecular masses. If the molecular mass of a compound is “x” then it represents that one molecule of that substance is x times heavier than 1/12(one-twelfth) of a carbon-12 atom.
Conclusion
Molecular mass and average molecular weight of a compound help determine the chemical composition of a compound and as the chemical composition of a compound explains the physical properties and chemical behaviour of the compound, thus the molecular mass and average molecular weight to an extent dictate the physical and chemical characteristics of the compound. “Molecular mass of a compound is the overall mass contained in a molecule of the compound whereas molecular weight is the force acting on a molecule of the compound”.
This article explained the concepts of molecular mass and average molecular weight of a compound in detail, both the terms are close in meaning and have only a slight difference.
