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Types of Chemical Formula

Chemical formula of an element is represented by the chemical symbol. In a compound containing more than one atom of the same element the number of the atom appears large before, or in small type below and after the chemical symbol.

In chemical formulas of a substance, the subscript numbers are used to indicate the number of atoms or groups of atoms of each element in the formula unit of the substance. The formula also provides information about the proportions in which atoms combine. Each of the chemical element is represented by chemical symbol and also the chemical formula provides specific information about the compound.

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Molecular Formula

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Molecular formula of a compound denotes the actual number of atoms of different elements present in one molecule of the compound. For determining the molecular formula, the molecular weight of compound should be known. Empirical formula weight is calculated by adding up atomic weights of various atoms present in the empirical formula of the compound. Molecular weight is then divided by empirical formula weight and the quotient is n. 

The formula for molecular formula is, 

Molecular formula = n $\times$ Empirical formula.
n = $\frac{Empirical \ formula}{Molecular \ formula}$.

The following are the steps to determine the molecular formula.
  • Determine the empirical formula of the compound.
  • Calculate the molar mass of the compound.
  • Determine the molar mass of the compound.
  • Divide the molar mass of the compound by the molar mass of the empirical formula unit to get n, the number of empirical formula units per molecule.
  • Write the molecular formula.

Empirical Formula

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The empirical formula of a substance shows atoms in a formula in their lowest ratio. A molecular formula of a substance shows the true compositions of a substance. The molecular formula of a compound is a multiple of its empirical formula. The molecular mass is some multiple of the empirical formula mass, which is obtained by summing the atomic masses of the atoms in the empirical formula. Once the empirical formula of a compound is determined, then the empirical formula mass can be calculated.

In order to find the empirical formula, the elements should be combined and their atomic masses and the ratio by mass or percentage in which they are combined. For any molecular compound empirical formula is written as, 

Empirical formula mass = $\frac{Molecular \ mass}{n}$.

Where, "n" is the number of empirical formula units in the molecule.

To determine the empirical formula just follow the steps given below.
  • Change percent to mass by assuming 100g sample.
  • Determine mole of each element by dividing the mass by the atomic mass of the element.
  • Find the mole ratio by dividing each mole by the smallest of the moles.
  • Write empirical formula using smallest whole number ratio.

Structural Formula

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The structural formula of a compound shows the linking pattern of various atoms in a molecule. In other words the two dimensional representation of organic molecules are called structural formulas. The two types of structural formulas are expanded structural formula and condensed structural formula.
  1. Expanded structural formula represents all atoms in a molecule and all the bonds connecting to each other.
  2. Condensed structural formula represents grouping of atoms in which the central atoms and the atoms connected to them are written as a group to show molecular structural formula.
  3. The skeletal structural formula shows the arrangement and bonding of carbon atoms present in the organic molecule and hydrogen is not shown with the carbon atoms.
The structural formula of some of the compounds of carbon is shown below. 

Structural Formula

Condensed Structural Formula

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A condensed structural formula is another naming method, one that straddles the divide between a molecular formula and a complete structure. For example, consider ethanol the functional group of ethanol that is hydroxyl group is bonded to a sp3 carbon atom. The condensed structural formula and the Lewis structure are represented below. 

Condensed Structural Formula

The CH3 in the condensed structural formula shows that the carbon is connected to three hydrogen atoms. Similarly the CH2 group shows that it is bonded to two hydrogen atoms and similarly the CH group is connected to one hydrogen atom. Always the shared pair of electrons is not shown in the condensed structural formula. The end carbon is bonded to only one other carbon atom, the other bonds are filled with hydrogen and are labelled as CH3 in condensed formula.
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