Calculations of Molecular formulae

There are many notations that are used by a chemist to determine and summarize atomic constituents in a symbolic way. These notations include empirical formula, molecular formula, and structural formula. The molecular formula includes the chemical symbols of the different elements along with their numerical values which show their composition in a molecule. Unlike the empirical formula which is the ratio between the lowest form of the whole number of the elements present in a molecule, it is a formula that provides the detail regarding the different elements present in the molecule with their atomic number. It is specific regarding the total number of atoms of different elements present in a molecule.

Molecular Formulas:

A molecule or compound is made up of two or more elements each contributing some atoms to it. Each element has its chemical symbol too. A molecular formula is a kind of formula or chemical representation of a compound or molecule which specifically gives information about each element present and the total number of atoms that contributed to the formation of the compound. 

Sometimes we observe that the empirical formula and the molecular formula of a component are the same. Here we can take the example of methane. The empirical formula of methane in CH4 and its molecular formula is also CH4. It is so because there is only 1 atom of carbon present in methane. However, it is just a coincidence, we must remember that the empirical formula of a compound is the ratio between the lowest whole number of atoms of elements whereas the molecular formula is the total number of atoms of the elements present in the molecule. It is the whole number multiplied by the empirical formula of a compound. For the purpose of determining the molecular formula of a molecule or compound, it is essential to calculate the molar mass of the given compound. 

A mass spectrometer is an instrument that is used by the chemist for determining the mass of a compound.

Molecular Weight :

The molecular weight of a molecule is the sum of the atomic masses of all the atoms present in the molecule. In other words, molecular weight is called molecular mass. The molecular weight of a molecule is based on 12. It is because 12 is the atomic weight of carbon. Due to isotopes of the elements, we find variation in the molecule masses of the same compound. The mass spectrometer is an instrument used by chemists to measure the mass of a molecule. This is also used to determine the composition of the elements in a molecule. Another method for determining the mass of a molecule is based on viscosity. 

Molecular Weight Formula:

Molecular weight is also known as molecular mass. In chemistry, we are provided the atomic weight of the molecules such as the atomic weight of carbon is 12, the atomic weight of hydrogen is 1. For the purpose of calculating the molar weight of a molecule, one must remember the atomic weight of each element that contributes to it. The atomic weights of the elements are found in the periodic table. It is a table having chemical elements which are arranged according to their atomic number. They are generally arranged in rows. There are 118 elements in the table. The first element of the table is hydrogen and the last element is oganesson (not sure, we can say as new elements are discovered day by day ). This table provides the atomic weight of the elements which are used for the calculation of molecular mass or molecular weight.

Steps for determining molecular mass:

• First of all, we should determine the molecular formula of the molecule.
• We identify each element present in the compound.
• Using the periodic table the atomic mass of each of the elements present in the molecule is noted.
• Finally, we have to multiply the atomic mass of each of the elements by the number of atoms of each element present in the compound.

Let’s solve an example for the calculation of molecular weight.

We have to calculate the molar weight of water. The chemical representation of water is H2O. Thus, in water, we have 2 atoms of hydrogen and 1 atom of oxygen. 

The atomic weight of hydrogen is 1

The atomic weight of oxygen is  16

Now for the purpose of calculating the molecular weight of water, we will add contribution from each atom of the elements present in the molecule.

2(1) + 1(16) = 18 grams/ mole

Molar Mass Formula:

The molar mass formula is the mass of the substances which is divided by the total number of substances.

M = m / n

Here, m = molar mass of a substance expressed in grams

           n = number of the moles of a substance

The SI. The unit of molar mass is kg/mol. It is also measured in g / mol.

Here, we can notice one thing that both the molar mass of carbon and atomic mass of carbon is 12. The only thing by which they can be differentiated is their measuring units. The atomic mass of carbon is measured in the unit of atomic mass and the molar mass of carbon is measured in the units of molar mass which is gram per mole. 

In the same way, atomic and molar masses of elements differ from each other by their measuring units. Molar masses and the molecular masses of the substances are also differentiated from each other by their units. Not only this the same condition applies with the formula masses of and molar masses of ionic compounds.

Conclusion

Thus, we can conclude that the molecular formula provides more details about the constituents of a compound than the empirical formula. It provides the total number of atoms of the elements present in a molecule. It is denoted with the chemical symbols of the elements present in a compound which is followed by the numeric subscripts which show the number of atoms of that particular substance present in the compound.

The molecular formula not only consists of chemical symbols of the molecules and the number of their moles present in a compound but also it sometimes presents some signs such as positive or plus (+), negative or minus(-), brackets, parentheses, dashes, etc.