Chemical Formula of Common Compounds– Importance of Chemical Formulae, Types, How to Write Chemical Formula, List, Practice Problems & FAQs

When we visit a restaurant we make it a point to genuinely go through the menu card to locate the food item that would best suit our appetite at that moment. But how do you choose your favourite dish? It is usually done by carefully checking out the condiments list used in making a particular recipe. When you know what goes into making a certain dish, you can correlate it to the probable taste that it would have and this is how you order your desired platter according to your taste!!

A chemical formula is quite similar to the cuisine details enlisted on a menu card. We refer to a chemical formula to understand the combination of elements and their proportions that constitute a certain compound. Just as we refer to the menu card to check for the combination of the constituting ingredients and condiments of a particular cuisine.

So let’s understand and learn more about how we obtain chemical formulae.

TABLE OF CONTENT

Chemical Formula–Definition
Importance of Chemical Formulae
Types of Chemical Formulae
Chemical Formula and Valency
Valency of Cations and Anions
How to Write Chemical Formula
List of Chemical Compounds with their Formulae
Practice Problems
Frequently Asked Questions– FAQs

Chemical Formula–Definition

A compound's chemical formula is regarded as a representation of its chemical make-up. A chemical formula describes the elements that make up a compound's molecules as well as the proportions in which their atoms combine to create those molecules. For instance, the chemical formula of water is H₂O. This implies that two hydrogen atoms and one oxygen atom are required to create a single water molecule. Chemical formulas provide light and distinction on a compound's chemical constitution.

The ratios in which the related elements combine to generate the desired compound are also represented by them. When a substance is represented in a chemical equation, its chemical formula is crucial. Ions, free radicals, and other chemical species can all be represented by chemical formulas.

Importance of Chemical Formula

Chemical equations give information about a compound's chemical constitution.
The ratios in which the component parts combine to form the compound are also represented by them.
When representing a compound in a chemical equation, its chemical formula is essential.
Ions, free radicals, and other chemical entities can all be represented by chemical formulae.
A formula is used to represent one chemical molecule.
In a formula, a substance's mass is indicated.

Types of Chemical Formulae

The compositions of chemical compounds can be stated in a variety of ways, as listed below. While the phrase "chemical formula" is typically used to refer to the molecular formula of a compound, which represents the total number of atoms of each constituent element in one complex molecule:

There are three types of chemical formulae.

Molecular formula: The amount of components that make up a chemical compound can be clearly seen by looking at the substance's molecular formula. The periodic table's symbols are used to represent the elements in molecular formulae, and a subscript is used to indicate how many atoms of each element there are in the molecule.

For instance, glucose has the chemical formula C₆H₁₂O₆, this indicates glucose is made of 6 atoms of carbon, 12 atoms of hydrogen and 6 atoms of oxygen.

Empirical Formula: A chemical compound's empirical formula reveals the different ratios of the elements it contains. Typically, empirical formulas are developed by the examination of experimental data.. The molecular formulas are used to create empirical formulations. A compound's molecular formula may be the same as its empirical formula or a multiple of it.

For example, CH₂O is the empirical formula for glucose. This indicates the whole number ratio of the atoms' that constitute glucose molecules, i.e., C, H, and O.

Molecular Formula= n (Empirical Formula)

The integer multiple n can be also obtained by dividing the molar mass of a compound by its empirical formula mass.

$n = \frac{M o l a r M a s s}{E m p i r i c a l F o r m u l a M a s s}$

Empirical formulae can be obtained from the percentage composition of each element constituting a compound and its overall molar mass. Steps involved here are:

Assuming the sample has a given overall mass (100 g is a decent mass to use when working with percentages), get the mass of each constituent.

Calculate the relative number of atoms of each element.

Calculate the simplest ratio of atoms for each element.

Use the simplest whole number ratio to write the empirical formula.

For example: A compound contains 6.57 % hydrogen, 40.12 % carbon and 53.31 % oxygen. Its empirical formula is:

Assuming the mass of the sample is 100 u.

Element	Mass Percentage	Mass of Elements in the Sample	Atomic Mass	Relative No. of Atoms	Simplest Atomic Ratio	Simplest Whole No. Ratio
Carbon	40.12 %	40.12 u	12 u	$\frac{40.12}{12} ≅ 3.34$	$\frac{3.34}{3.34} = 1$	1
Hydrogen	6.57 %	6.57 u	1 u	$\frac{6.57}{1} = 6.57$	$\frac{6.57}{3.34} ≅ 2$	2
Oxygen	53.31 %	53.31 u	16 u	$\frac{53.31}{16} ≅ 3.34$	$\frac{3.34}{3.34} = 1$	1

So the empirical formula is CH2O. If molecular weight or molar mass is known, we can find n and hence obtain its molecular formula too.

Structural Formula: A chemical compound's structural formula gives a clear picture of how its atoms are arranged in the molecule.Although molecular formulas are concise and simple to express, they do not describe atomic arrangement,or bonding of atoms. We require the structural formula for it. The atomic count and spatial arrangement are both given by the structural formula.

For example: Structure of glucose can be represented as:

Chemical Formula and Valency

As we have seen, atoms are joined together to form molecules. The ability of an atom to combine varies between elements. This is referred to as valency. The ability of an element to combine with other elements is known as valency. It is measured in terms of the quantity of hydrogen atoms, chlorine atoms, or two times as many oxygen atoms as that element's own atom. For example:

In the molecule PH3, the valency of the element phosphorus (P) is 3.

In the molecule SF6, the valency of sulphur (S) is 6.

Along with the atoms, the charged species known as ions also have some valencies. Positive valencies are found in positive ions and cations ( ${N a}^{+}, {C a}^{2 +}, {F e}^{3 +}$ etc.). Negative valencies are found in negative ions or anions (e.g. ${C l}^{-}, {C O}_{3}^{2 -}, {B r}^{-}, {S O}_{4}^{2 -}$ etc.).

Valency of Cations & Anions

Depending on the charge they have, positive ions can be monovalent, divalent, trivalent, tetravalent, etc. Cations having more than one atom in them are called polyatomic cations. The following table includes a list of them:

Cations Name	Cation	Valency
Hydrogen	H⁺	1
Lithium	Li⁺	1
Sodium	Na⁺	1
Potassium	K⁺	1
Calcium	Ca²⁺	2
Silver	Ag⁺	1
Iron (Ferrous)	Fe²⁺	2
Iron (Ferric)	Fe³⁺	3
Lead (Plumbous)	Pb²⁺	2
Lead (Plumbic)	Pb⁴⁺	4
Chromium	Cr³⁺	3
Antimony	Sb³⁺	3
Phosphonium	PH⁴⁺	1
Ammonium	NH⁴⁺	1
Mercury (mercurous)	Hg⁺	1
Mercury (mercuric)	Hg²⁺	2

Negative ions can occur in nature as monovalent, divalent, trivalent, etc. ions much like positive ions. Anions having more than one atom constituting them are called polyatomic anions. Below is a list of these as well:

Elements	Name of Anions	Anions	Valency
Fluorine	Fluoride	F^-	1
Bromine	Bromide	Br^-	1
Chlorine	Chloride	Cl^-	1
Oxygen	Oxide	O^2-	2
Nitrogen	Nitride	N^3-	3
Phosphorus	Phosphide	P^3-	3
Hydrogen	Hydride	H^-	1
Carbon	Carbide	C^2-	2
Sulphur	Sulphide	S^2-	2
Silicon	Tetrasilicide	Si^44-	4

Here is a list of some polyatomic anions:

Polyatomic Anions’ Name	Anions	Valency
Hydroxide	OH^-	1
Carbonate	${C O}_{3}^{2 -}$	2
Hydrogen carbonate/Bicarbonate ion	${H C O}_{3}^{-}$	1
Sulphate	${S O}_{4}^{2 -}$	2
Hydrogen Sulphate	${H S O}_{4}^{-}$	1
Thiocyanate	SCN^-	1
Isothiocyanate	NCS^-	1
Manganate	${M n O}_{4}^{2 -}$	2
Permanganate	${M n O}_{4}^{-}$	1
Sulphite	${S O}_{3}^{2 -}$	2
Nitrate	${N O}_{3}^{-}$	1
Nitrite	${N O}_{2}^{-}$	1
Peroxide	$O_{2}^{2 -}$	2
Phosphate	${P O}_{4}^{3 -}$	3

How to Write Chemical Formula

Knowing the symbol of each element present in the compound, the radical formula, and the valency of each element are necessary to write a chemical formula. A formulation of the chemical compounds is put together according to a set of rules and regulations. The following list of regulations includes some of them:

The majority of naturally occurring compounds are binary compounds, meaning that they only include two components. However, there are also naturally occurring chemical compounds having more than two elements. Examples: $H_{2} S, {C a C O}_{3}, {N a A l F}_{6}$ .

Ions are the name for the polarised elements or atoms. A positive-charged ion is referred to as a cation, whereas a negative-charged ion is referred to as an anion.

The metal is named first followed by the non-metal for chemical compounds that are binary compounds made up of a metal and a nonmetal. Take NaCl, as an example, also known as sodium chloride, which is composed of the ions Na+ (a metal ion) and Cl- (a non-metal ion).

Anions with a -1 negative charge typically contains the suffix -ide in all known ions. Cl-, for instance, is known as chloride.

Oxyanions, which are composed of oxygen and another element, typically contain the suffix -ate. For instance, SO₄^2- is known as sulphate.

When a polyatomic anion contains a hydrogen, bi- or hydrogen is used. HCO₃^-, for instance, is also known as bicarbonate or hydrogen carbonate.

Now let’s see, how the formulae of compounds are written using the following method:

1. The positive atom or cation is written on the left side of the constituent element symbol, while the negative atom or anion is written on the right side.

2. The valency numbers become subscripts of the associated symbols, and the valencies of the symbols are printed below them and shifted crosswise to the lower right corner of the symbols.

3. If necessary, the valency numbers are divided by a common factor to provide a simple ratio. Let's understand this using the example of calcium chloride.

Example of representation of chemical formula:

Calcium chloride is made up of two elements i.e., calcium and chlorine. Calcium being a metal of group II forms a divalent cation, Ca²⁺. Chlorine is a halogen and forms chloride anion Cl-. Now we cross-over the valencies of calcium and chloride ions to obtain the final chemical formula of this compound.

So, we get CaCl₂. This denotes the compound is made of two atoms of chlorine and one atom of calcium.

List of Chemical Compounds & their Formulae

Here is a list of some chemical compounds along with their chemical formulae:

Compound Name	Chemical Formula
Ammonium Hydroxide	NH₄OH
Methane	CH₄
Ammonium Dichromate	NH₄2Cr₂O₇
Barium Sulphate	BaSO₄
Boron trifluoride	BF₃
Phosphoric Acid	H₃PO₄
Barium Fluoride	BaF₂
Barium Hydroxide	Ba(OH)2
Potassium Bromate	KBrO₃
Calcium Phosphate	Ca₃PO₄²
Dinitrogen Pentoxide	N₂O₅
Potassium hydrogen phosphate	KH₂PO₄
Potassium Bromate	KBrO₃
Lead Acetate	PbCH₃COO₂
Magnesium carbonate	MgCO₃
Potassium dichromate	K₂Cr₂O₇
Potassium permanganate	KMnO₄
Sodium peroxide	Na₂O₂
Titanium Dioxide	TiO₂
Lead Sulphate	PbSO₄
Zinc hydroxide	ZnOH₂
Zinc sulphide	ZnS

Practice Problems

Q.1 What is the molecular formula of elements? Give Examples.

Answer: The symbol for an element and the subscript for the number of atoms in a single molecule serves as a description of the chemical composition of the molecule in an element's chemical formula.

For example: Chemical formula of the element oxygen is O₂. This is also known as the molecular formula of oxygen. This denotes that one molecule of oxygen is composed of two atoms of oxygen. Hence oxygen is a diatomic molecule.

Q.2. The chemical formula for magnesium sulphide is:

Mg₂S₃
MgS₂
Mg₂S
MgS

Answer: (D)

Solution:

Cation and anion of magnesium and sulphide are denoted as: Mg²⁺ and S^2- respectively. Hence they both have equal and oppositely charged valencies. So their formula is MgS.

Q.3. The chemical formula for ammonium hydroxide is:

NH₄OH
NH₃OH
NH₄(OH)₃
N(OH)₄

Answer: (A)

Solution:

Ammonium ion is depicted as a cation NH4+ and hydroxide ion is a polyatomic anion i.e, OH-.

So their formula is NH₄OH as they have the same and opposite valency. So option A is the correct answer.

Q.4. Sodium carbonate is written as:

NaCO₃
Na₂CO₃
NaHCO₃
Na(CO₃)₂

Answer: (B)

Solution:

Sodium ion is Na+ and carbonate ion is written as CO₃^2- . Now following the rule of valency crossover, we shall write the valency of carbonate as subscript of Na and valency of Na as a subscript of carbonate ion. So the answer is Na₂CO₃.

Frequently Asked Questions– FAQs

Q.1. Naturally what form do the noble gases exist in?
Answer: Noble gases are chemically inert and hence they are unreactive in nature. They tend to naturally exist in monatomic states in nature. So their chemical formulae are similar to their elemental symbol itself.

Q.2. What is the difference between O₂ and 2O?
Answer: O₂ is the molecular formula of oxygen. In this, the two atoms constituting oxygen are covalently bonded to each other forming an oxygen molecule. O₂ symbolises two separate atoms of oxygen in their elemental state.

Q.3. What is the requirement of having a chemical formula?
Answer: Each constituent element in a chemical formula is identified by its chemical symbol, along with the relative number of atoms that make up each element.

Q.4. Why do different isotopes of the same element exhibit the same chemical formula?
Answer: Chemical formula is a chemical property that is determined by the number of combined atoms constituting a compound. Because they contain the same number of electrons as an atom of that element, but varying quantities of neutrons, all isotopes of an element have the same chemical characteristics. The mass number is impacted by the varied neutron counts. The physical and chemical qualities are determined by the mass number and the atomic number, respectively.

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