Molecular Weight Formula

What is Molecular Weight?

The molecular weight definition can be given as the sum of the atomic weight values of the atoms in a given chemical compound. Thus, the molecular weight meaning clearly states that it is the total of all the atomic weights of each of the atoms present in a given molecule of a chemical compound. In case of single elements, the molecular weight is the sum of the atomic weights of the total number of atoms of an element required to form a stable molecule. For example, in the case of O2, the molecular weight is 32, which is the sum of the atomic weight of a single Oxygen atom. Similarly, the molecular weight of Fe i.e. iron is 56, since it is a single atom. 


How to Calculate Molecular Weight Formula?

The molecular weight of a given molecule is the sum of the atomic weights of all the atoms present in a given molecule. This information can be easily obtained from the molecular formula. Thus, the molecular weight definition and formula can both be derived from the molecular formula of a given molecule. The number of each type of atom present in a molecule is multiplied by the atomic weight and is then added to the weights of other atoms. Thus, in simple terms, the molecular weight formula is nothing but the sum of the total number of atoms present in a given molecule which can be known from the given molecular formula. 

Typically, the molecular weight formula gives the value in units of amu or Da. Here, amu is the atomic mass unit and Da is Dalton. Both the atomic weight and the molecular weight are defined with respect to the mass of isotope of carbon - 12. This isotope has an assigned value of 12 amu and is thus regarded as standard. The molecular weight in turn helps you in knowing how many grams of a chemical substance is present in one mole of a compound. The mole in turn is used to determine the stoichiometry of a given molecule in chemical reactions and equations. Thus, molecular weight helps in determining the amount of a chemical compound required for a given chemical reaction. 

One point of note is that, there are minor differences between molecular weight, formula weight and molecular mass. As given above for you how to calculate molecular weight, it is clear that it can be done using the molecular formula of the chemical substance. And generally the weight obtained from the chemical formula or molecular formula is both the formula weight and the molecular weight of the substance. But in the case of ionic substances, the formula weight is different from the molecular weight. The weight usually obtained from the chemical formula of ionic substances is the formula weight. This is because the ionic substances do not have chemical bonds and do not exist as individual molecules. They dissociate as soon as they are dissolved into a solution. Therefore, for ionic substances, the molecular weight cannot be obtained and the formula weight is generally taken as the molecular weight. Also, the difference between molecular weight and molecular mass is a matter of simple definition of weight. The weight of any given substance is the force (typically gravitational force) acting on the given substance having a specific mass. Hence, by this concept molecular weight is the force acting on a given chemical substance having a molecular mass. But these forces usually are very minimal and are not applicable at such a small scale and hence, molecular weight and molecular mass are used interchangeably. 


Given Below are two Examples About How to Calculate Molecular Weight

Example 1: How to Find the Molecular Weight of C6H6?

Solution: From the given molecular formula it is clear that the given chemical substance has 6 carbons and 6 hydrogen atoms. Now, the atomic weights of carbon and hydrogen atoms are 12.0107 amu and 1.008 amu respectively.       

Thus, the molecular weight of C6H6 is  = 6*(atomic weight of carbon) + 6*(atomic weight of hydrogen)

        = 6*(12.0107 amu) + 6*(1.008 amu)

                    =  72.0642 amu + 6.048 amu

                    = 78.112 amu. 


Example 2: How to Find the Molecular Weight of Fe2O3?

Solution: From the given molecular formula it is clear that the given chemical substance has 2 iron atoms and 3 oxygen atoms.      

Now, the atomic weights of iron and oxygen atoms are 55.847 amu and 15.9994 amu respectively.      

Thus, the molecular weight of Fe2O3 is = 2*(atomic weight of iron) + 3*(atomic weight of oxygen)

        = 2*(12.0107 amu) + 3*(1.008 amu)

                    = 159.692 amu. 

FAQs (Frequently Asked Questions)

1. How to Find the Molecular Weight of a Given Molecule?

Ans: The molecular weight of any given molecule is the sum total of the atomic weights of each of the atoms present in making the molecule. The information about the total number of each atom present in the molecule can be known from the molecular formula. Hence, from the molecular formula of a given molecule one can easily find the molecular weight. 

2. What is Formula Weight and Molecular Weight?

Ans: The formula weight of a given molecule is the sum of the atomic weights of all the atoms present in the empirical formula of the molecule. On the other hand, the molecular weight of the molecule is the sum of the atomic weights of all the atoms as per the molecular formula. 

3. What is the Unit of Molecular Weight?

Ans: Molecular weight is usually expressed in terms of amu or Da. Here, amu stands for atomic mass unit and Da for dalton. Usually in chemical reactions or while analysing the amount required of a molecule for a chemical reaction, the unit of molecular weight used is gram/ mole or kilogram/mole.