Molecular Formula
Molecular Formula
A molecular formula uses chemical symbols to represent the elements in a molecule, along with numeric subscripts indicating the number of atoms of each element. It conveys information about the proportions of the different elements in the molecule. Learn more about molecular formula in this article by Extramarks.
What is Molecular Formula?
A molecular formula represents the total number of individual atoms in a compound’s molecule. It uses subscripts to indicate the quantity of each type of atom in a molecule of the substance. Molecular formulas are related to gram molecular masses, which are whole number multiples of the empirical formula mass. This formula specifies the type and number of atoms in a single chemical molecule, using chemical symbols for the elements with subscripts showing the number of atoms of each element present in the molecule.
Expression of Molecular Formula
The molecular formula is expressed as n(Empirical formula), where it indicates the type and number of atoms of each element in a molecular compound. This formula is always an integer multiple of the empirical formula and represents the whole-number ratio of atoms in the compound. The molecular formula of a compound can either be the same as or a multiple of its empirical formula.
N = Molecular Formula/ Empirical Formula
What is the Empirical formula?
The empirical formula is the simplest formula for a compound, showing the ratio of the smallest whole number of each element’s subscripts. Also known as the most basic formula, it expresses the composition with minimal integer subscripts. The empirical formula indicates the number of atoms in a specific ratio and is directly proportional to the compound’s percent composition. Unlike the molecular formula, the empirical formula shows the ratio of elements in the compound rather than the actual number of atoms in each molecule. The subscripts next to the element symbols represent these ratios.
Calculation of Molecular Formula
To determine the molecular formula of a compound, it involves several steps:
- Determine the Empirical Formula:
- Obtain the mass percentage composition of each element in the compound.
- Convert these percentages to moles by dividing by each element’s atomic mass.
- Divide the mole values by the smallest number of moles obtained. Round to the nearest whole number to determine the simplest ratio of atoms, which provides the empirical formula.
- Calculate the Empirical Formula Mass:
- Add up the atomic masses of all the atoms in the empirical formula to determine their total mass.
- Determine the Molar Mass of the Compound:
- This is generally measured experimentally using methods such as mass spectrometry.
- Calculate the Molecular Formula:
- Divide the compound’s molar mass by the empirical formula mass to obtain a whole number, n.
Multiply all the subscripts in the empirical formula by n to derive the molecular formula.
Law of Composition
The law of composition states that elements always combine in the same ratio to one another in any particular chemical compound. According to the law of constant composition, every sample of a given chemical compound will contain the same elements in the same ratio or proportion. For instance, any water molecule is typically constructed in a 2:1 ratio using two hydrogen atoms and one oxygen atom. If we take a quick look at the relative masses of oxygen and hydrogen within a water molecule, we can observe that oxygen makes up 94% of the molecule’s mass. Therefore, the mass of hydrogen makes up the remaining 6%. As a result, this mass ratio will be comparable to any water molecule.
Solved Examples for Molecular Formula
Example: The empirical formula of a compound containing boron and hydrogen is BH₃. Its molar mass is 27.7 g/mol. Determine the molecular formula.
Solution:
Empirical formula molar mass (EFM) = 13.84 g/mol.
Divide the molar mass of the compound by the empirical formula mass.
Molar mass/EFM = 27.7g/mol/13.84g/mol = 2 EFM
Multiply all the subscripts in the empirical formula by the whole number n
BH3×2=B2H6
FAQs (Frequently Asked Questions)
1. What does a molecular formula represent?
A molecular formula provides the specific number of each type of atom within a molecule of a compound, offering insight into both the types and quantities of atoms present.
2. What steps are involved in determining the molecular formula?
The process involves finding the empirical formula, calculating its molar mass, then dividing the compound’s molar mass by that of the empirical formula. The resulting whole number (or close approximation) is used to adjust the subscripts in the empirical formula to obtain the molecular formula.
3. How do empirical and molecular formulas differ?
While the empirical formula displays the simplest whole-number ratio of atoms in a compound, the molecular formula provides the precise count of each type of atom within a molecule. Additionally, the empirical formula might be a multiple of the molecular formula.
4. Is it possible for a compound to possess multiple molecular formulas?
Yes. A compound can show multiple molecular formulas if its empirical formula can be scaled by different whole numbers to give distinct molecular formulas with identical overall compositions.
