Molecular Formula: Definition, Formula and Steps for Writing

Chemical formulas were first derived by determining the masses of all the elements that combine to create a molecule, leading to the development of two important types of chemistry formulae: molecular formula and empirical formula. The molecular formula gives the exact number of every single atom present in a molecule, whereas the empirical formula gives the simplest ratio of the number of various atoms present. It is an empirical formula if the formula is simplified. The molecular formula is a multiple of the empirical formula and is often used.

Key Terms: Definition, Meaning, Difference, Steps for writing for Empirical and Molecular Formula

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What is the Molecular formula?

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The molecular formula is a formula generated from molecules that represents the total number of individual atoms in a compound's molecule. A subscript in a molecular formula indicates the number of each type of atom in a molecule of the substance. Molecular formulas are linked to gramme molecular masses, which are simple whole number multiples of the empirical formula mass.

It specifies the quantity and kind of atoms in a single chemical molecule. The chemical symbols for the constituent elements are used, and the number of atoms of each element present in each molecule is provided as a subscript after that element's symbol. 

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What is the Empirical formula?

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The empirical formula is the shortest formula for a compound, described as the ratio of subscripts of the least whole number of elements in the formula. It's also referred to as the most basic formula. The formula of a component expressed with the minimal integer subscript is called an empirical formula. The empirical formula specifies the number of atoms in the compound in a given ratio. The empirical formula of a compound is directly proportional to its % composition.

The empirical formula of a compound is the formula that shows the ratio of components contained in the compound, rather than the actual number of atoms found in the molecule. Subscripts adjacent to the element symbols indicate the ratios.

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Empirical Formula Vs Molecular Formula

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Determination of Molecular Formulae

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The molecular formula of a compound is the formula that specifies the number of atoms of various elements present in the compound's molecule. The chemical glucose, for example, has the molecular formula C₆H₁₂O₆. A molecule of glucose has six carbon atoms, twelve hydrogen atoms, and six oxygen atoms. The first step in determining a compound's molecular formula is to calculate its empirical formula from the percentage composition.

The empirical formula of a compound is the formula that yields the simplest whole-number ratio of atoms among the many elements contained in the compound's molecule. The empirical formula for glucose (C₆H₁₂O₆) is CH₂O, indicating that C, H, and O are present in the simplest ratio of 1: 2: 1 in the chemical. 

Formula based on evidence A substance's mass is equal to the sum of the atomic masses of all the atoms in its empirical formula.

The empirical formula is multiplied by a whole number in the molecular formula. Thus Molecular formula = (Empirical formula) ×n where n = 1, 2, 3

\(n = \frac{Molecular formula}{Empirical formula} = \frac{Molecular mass}{Empirical formulamass}\)

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Finding Molecular Formulas Using Empirical Formulas

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If you know the molar mass of the chemical, you may apply the empirical formula to obtain the molecular formula. Calculate the empirical formula mass and then divide the compound molar mass by the empirical formula mass to get the empirical formula mass. The ratio between the molecular and empirical formulas is obtained in this way. To get the subscripts for the molecular formula, multiply all of the subscripts in the empirical formula by this ratio.

Steps for writing the empirical formula

The proportion of the elements in a compound is determined using appropriate procedures, and the empirical formula is determined using the data gathered in the following steps:

• Divide the atomic mass of each element by the percentage. This will tell you how many moles of each element are present in the compound.
• To get a simple ratio of moles of various elements, divide the quotients obtained in the previous step by the smallest of them.
• If required, multiply the figures by a suitable integer to achieve a whole number ratio.
• If required, multiply the figures by a suitable integer to achieve a whole number ratio.
• Lastly, put the characters for the various components side by side, with the above numbers as subscripts in the lower right-hand corner of each symbol.
• The empirical formula of the compound will be represented by this.

Steps for writing the molecular formula

• Assemble the empirical formula using the steps outlined in this article.
• Calculate the empirical formula mass by summing the atomic masses of all the atoms in the compound's empirical formula.
• Find the value of n by dividing the molecular mass (measured empirically using a suitable method) by the empirical formula mass.

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Things to Remember

• In contrast to ionic compounds, a molecular compound's formula cannot be simply calculated based on its constituent elements because the same elements can produce a variety of molecular compounds, each with its own formula.
• The relative amount of each element in a compound may be determined using a chemical formula and the molar masses of its constituent elements, which is incredibly relevant information.
• The mass percent composition of an element in a compound is one way to represent how much of that element is present in that compound. The mass percent composition, or simply the mass percent, of an element, is its percentage of the total mass of the compound.
• If you also know the molar mass of the chemical, you can calculate the molecular formula from the empirical formula. The empirical formula is always multiplied by a whole number in the molecular formula.

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Previous Year Questions

• Unusual high boiling point of water is result of​
• Two ice cubes are pressed over each other until they unite to form one block. Which one of the following forces dominates for holding them together ?
• According to VSEPR theory, the shape of the water molecule is​
• According to Fajans rule covalent bond is favoured by:​
• Which of the following represents the arrangement in increasing order of bond order and bond dissociation energy?
• ​Correct order of bond angles for the following is
• Out of N2O,SO2,I+3,I−3,H2O,NO−2andN−3N2O,SO2,I3+,I3−,H2O,NO2−andN3−the linear species are​
• Formation of \cePCl3\cePCl3 is explained on the basis of what hybridisation of phosphorus atom​
• In the change of NO→NO+NO→NO+ the electron is added to
• In SOCl2SOCl2, the Cl−S−ClCl−S−Cl and Cl−S−OCl−S−O bond angles are​
• In which of the following, there are 3 bp and 2 lp?
• Identify the correct sequence of increasing number of a bonds in the structures of the following molecules: I. H2S2O6H2S2O6 II. H2SO3H2SO3 III. H2S2O5​
• The ground state magnetic property of B2B2 and C2C2 molecules will be​
• Dipole moments of HCI = 1.03 D, HI= 0.38 D. Bond length of HCl=1.3˚AHCl=1.3A˚ and HI=1.6˚AHI=1.6A˚. The ratio of fraction of an electric charge, e existing on each atom in HCl and HI is​
• Which of the following molecules has zero dipole moment?​
• Which of the following does not contain any coordinate bond?​
• What is the geometry of water molecule?​
• The type of bonds present in sulphuric anhydride are​