Writing Chemical Formula of a Compound: A Basic Guide

Chirag Gupta

3 years ago

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To know the concept of writing chemical formula of chemical compound, you should know the following:

Symbols of the combining elements
Valency of the combining elements

Name, Representation and Valency of ions

Name, representation and valency of some common ions are given below:

Note: You have to remember the valencies given in the above table.

Valency to remember: https://www.youtube.com/shorts/Y7jf_Gt5Hg4?feature=share

Writing Formula of Molecular Compounds

How is the chemical formula of a molecular compound written?

The chemical formula of a molecular compound is written as follows:

Step 1: Write the symbols of the constituent elements side by side, in such a way that the less electronegative element is on the left and the more electronegative element is on the right.

Step 2: Write their valency numbers over the symbols and criss-cross the valency numbers to write as subscripts to the symbols.

Step 3: Divide these numbers, written as subscripts, by a common factor, if needed.

This method of writing the chemical formula of a compound is illustrated as:

Water contain hydrogen and oxygen. Oxygen is more electronegative than hydrogen. So,

Step 1: Write the symbol of hydrogen and oxygen side by side, with symbol of oxygen on the right.

H O

Step 2: Write the valency of hydrogen and oxygen in water.

H(+1) O(-2)

Step 3: Criss-cross the valency numbers.

H₂O₁ or H₂O is the formula of water.

Hydrogen chloride contains hydrogen and chlorine. Chlorine is more electronegative than hydrogen. Then by the following the steps:

Step 1: H Cl

Step 2: H(+1) Cl(-1)

Step 3: HCl is the formula of hydrogen chloride

Writing formula of ionic compounds

How to write the stoichiometric formula of ionic compound?

The ionic compounds contain ions. These ions are arranged in such a way that the cation has an anion as its nearest neighbour and vice versa. There is no discrete molecule of an ionic compound, therefore, it is not correct to assign a molecular formula to an ionic compound.

Instead, an ionic compound is described by a formula which describes a simple atomic ratio of the elements present in it. The formula which describes the simplest atomic ratio of the elements present in a compound is called its stoichiometric formula. For example, sodium chloride is an ionic compound in which sodium and chlorine are present in 1:1 ratio. So, the stoichiometric formula of sodium chloride is NaCl.

To write the stoichiometric formula of an ionic compound, proceed as follows:

Step 1: Write the symbol of the cation showing the charge number on it. Write the symbol of the anion showing the charge number on it, on the right hand side of the cation.

Step 2: If a compound contain polyatomic ions, the formula of the iron is enclosed within brackets before criss-crossing the valencies.

Step 3: Now, write the charge number (valency) of the cation at the bottom right of the anion and the charge number of anion at the bottom right of the cation. Thus the symbol of cation is subscripted with the charge number of anion and the anion is subscripted with the charge number of cation. This is called the criss-crossing of valancies.

Step 4: If these subscripts are 1, then these are not written in the final stoichiometric formula. Otherwise, these subscripts are reduced to the lowest possible whole number by dividing each by the highest common factor.

The method of writing the chemical formula of ionic compound is illustrated through the following examples.

Aluminium sulphate: Write symbols of ions side by side Al(3+) SO₄(2-). The formula is Al₂(SO₄)₃.

Molecular mass

What is molecular mass of a substance?

The molecular mass of a substance is equal to sum of atomic masses of all the atoms present in a molecule of that substance. The molecular mass of substance may be defined as, “the average mass of a molecule of a substance is expressed in atomic mass unit is termed as its molecular mass”.

How is molecular mass of a substance calculated?

Molecules are composed of two or more atoms of the same or different elements. Mass is an additive property. So, the molecular mass of any substance can be obtained by adding together the atomic masses of all the atoms present in one molecule of a substance.

For example, the molecular formula of water is H₂O.

Atomic mass of hydrogen = 1 u

Atomic mass of oxygen = 16 u

Molecular mass of H₂O = (2*Atomic mass of hydrogen)+(1*Atomic mass of oxygen) = (2*1)+(1*16) = 18 u

Formula Mass

What is formula mass of a substance?

For ionic compounds, such as sodium chloride, there is no discrete individual molecule. Instead, these are aggregates represented by a formula of the type, (P⁺B^–)_n.

Solid sodium chloride is represented by the formula (Na⁺Cl^–)_n. Thus, the simplest formula for the sodium chloride can be written as NaCl. So, the concept of formula mass is used for ionic compounds.

The sum of atomic masses of all atoms in a formula unit of a substance is called the formula mass of the substance.

How is the formula mass of an ionic substance calculated?

The formula mass of any ionic compound is expressed as the sum of the atomic masses of all the atoms present in its formula. For example, for NaCl,

Atomic mass of Na = 23 u

Atomic mass of Cl = 35.5 u

Formula mass of NaCl = (1*Atomic mass of Na)+(1*Atomic mass of Cl) = (1*23)+(1*35.5) = 58.5 u

Thus, the molecular mass, or empirical formula mass, of a substance is equal to the sum of the atomic masses of all the element present in its molecular formula, taking into account the number of atom of each element present in its molecular formula.

Composition of a compound

A compound contains two or more elements combined in a certain fixed ratio. The composition of a compound can be described by the mass percentage of each element present in it.

How to determine the mass percentage of an element in a compound?

Composition of compound is commonly expressed as the mass percentage of each element present in it. To determine the mass percentage of an element in any compound, we should know the mass of that element present in a certain known mass of the compound.

Let us suppose, W g of compound contain w g of an element A. Then,

To do this, a known mass of the compound is chemically analysed, and the mass of each element present in it is determined by a suitable chemical method.

For example, the masses of hydrogen and oxygen in water can be determined by electrolysing water. The volumes of hydrogen and oxygen obtained during electrolysis are measured, and thus their masses calculated.

How is the percentage composition of a compound determined from its formula?

The percentage composition of a compound means the mass percentage of each element present in the compound. The mass percentage of each element can be determined from the molecular or empirical formula of the compound as follows:

Step 1: Write the molecular or empirical formula of the compound.

Step 2: Write the name, number of atoms and total mass of each element present in one molecule or one formula unit of the compound.

Step 3: Obtain the molecular mass or formula mass of the compound by adding the masses of all the element present in it.

Step 4: Calculate the mass percentage of each element by using the relationship.

Example: Molecular formula of carbon dioxide is CO₂. Find the molecular mass and mass percentage of each element in carbon dioxide.

Solution: Carbon dioxide has 1 carbon atoms and 2 oxygen atom.

Atomic mass of carbon = 12 u