Mole Concept

1 What is the Mole Concept?

The mole concept is a convenient method of expressing the amount of a substance. Any measurement can be broken down into two parts – the numerical magnitude and the units that the magnitude is expressed in. For example, when the mass of a ball is measured to be 2 kilograms, the magnitude is ‘2’ and the unit is ‘kilogram’.

When dealing with particles at an atomic (or molecular) level, even one gram of a pure element is known to contain a huge number of atoms. This is where the mole concept is widely used. It primarily focuses on the unit known as a ‘mole’, which is a count of a very large number of particles.

2 What is a Mole?

In the field of chemistry, a mole is defined as the amount of a substance that contains exactly 6.02214076 * 1023 ‘elementary entities’ of the given substance.

The number 6.02214076*1023 is popularly known as the Avogadro constant and is often denoted by the symbol ‘NA’. The elementary entities that can be represented in moles can be atoms, molecules, monoatomic/polyatomic ions, and other particles (such as electrons).

For example, one mole of a pure carbon-12 (12C) sample will have a mass of exactly 12 grams and will contain 6.02214076*1023 (NA) number of 12C atoms. The number of moles of a substance in a given pure sample can be represented by the following formula:

n = N/NA

Where n is the number of moles of the substance (or elementary entity), N is the total number of elementary entities in the sample, and NA is the Avogadro constant.

The word “mole” was introduced around the year 1896 by the German chemist Wilhelm Ostwald, who derived the term from the Latin word moles meaning a ‘heap’ or ‘pile.

4 Related Quantities and their Formulae

Atomic and Molecular Mass

The atomic mass of an element is the mass of one atom of the element expressed in atomic mass units (amu). It accounts for the abundance of the various isotopes of the element and assigns an average value to the mass of one atom of the element.

For example, the atomic mass of carbon is 12.011 atomic mass units since carbon samples generally contain 98.89% of the carbon-12 isotope, 1.11% of carbon-13, and trace amounts of carbon-14. However, the atomic masses of these isotopes are different.

The atomic mass of a carbon-12 atom is 12 atomic mass units, but that of a carbon-13 atom is 13 amu. The atomic mass of an element is roughly equal to the sum of all the protons and neutrons present in its nucleus.

The molecular mass of an element is the sum of the atomic masses of all its constituent elements. This quantity is also represented in terms of atomic mass units. Therefore, the molecular mass of water is equal to the sum of the atomic masses of its constituents – hydrogen and oxygen.

The atomic mass of hydrogen is 1.00794 amu and that of oxygen is 15.9994. Since water molecules contain 2 hydrogen atoms and only one oxygen atom, the molecular mass of H2O is 18.0154 amu.

4 Molar Mass

The molar mass of a substance is defined as the total mass of one mole of the substance. It is often represented in terms of ‘grams per mole’ (g/mol). However, the SI unit of this quantity is kg/mol. Molar mass can be represented by the following formula:

Molar mass of a Substance = (Mass of the Substance in grams)/(Number of Moles)

For example, the molar mass of water is approximately 18.015 g/mol, which is the mass of NA number of water molecules.

5 Gram Atomic Mass and Gram Molecular Mass

The gram atomic mass of an element is the mass of one mole of that element. Similarly, the gram molecular mass of a compound refers to the mass of a single mole of the compound. Therefore, the gram atomic mass of hydrogen is approximately 1.007g and the gram molecular mass of water is approximately 18.015g.

6 Related Formulae

The number of moles in a given sample of an element/compound can be calculated by dividing the total mass of the sample by the molar mass of the element/compound, as described by the following formula.

Number of Moles = (Mass of the Sample)/(Molar Mass)

The total number of atoms/molecules in a sample can be calculated by multiplying the number of moles with the Avogadro constant. This formula can be written as:

Number of Atoms or Molecules = (Number of Moles)*(6.022*1023)

The relationship between the atomic mass unit (amu) and the gram is given by:

1 amu = (1gram)/(6.022*1023) = 1.66*10-24 grams

Therefore, the mass of one mole of an element will be equal to its atomic mass in grams.

7 Solved Examples on the Mole Concept

Some solved example questions on the mole concept are provided in this subsection.

Q.1: How many moles of iron are present in a pure sample weighing 558.45 grams?

A.1: The molar mass of iron is 55.845 g/mol. Therefore, the number of moles of iron in the pure sample weighing 558.45 grams is:

= 10 moles.

Q.2: How many molecules of water are present in 36 grams of water?

A.2: The molar mass of water is 18 (approximately). Therefore, 36 grams of water makes up a total of 2 moles. Each mole has 6.022*1023 water molecules. The total number of H2O molecules in 36 grams of water is: 12.044*1023

Q.3: How many grams of carbon can be found in 1 mole of carbon dioxide?

A.3: 1 mole of CO2 contains 1 mole of carbon and 2 moles of oxygen. The molar mass of carbon is 12.0107 g/mol. Therefore, 1 mole of CO2 contains 12.01 grams of carbon and 32 grams of oxygen.

Frequently Asked Questions – FAQs

What is a mole equal to?

One mole of a substance is equal to the substance’s 6,022 x 1023 units (such as atoms, molecules, or ions). The 6.022 x 1023 number is known as the number of Avogadro or the constant of Avogadro. For the conversion of mass and number of particles, the definition of the mole can be used.

What is the importance of the mole concept?

All chemistry is pervaded by the mole definition. Since most quantitative chemical calculations are focused on the mole, for the study of chemistry, an understanding of the mole is important. A knowledge of how the mole applies to mass, number of entities

What is a mole in chemistry?

In chemistry, the mole, also written mol, is a standard scientific unit for the calculation of large amounts of very small entities, such as atoms, molecules, or other objects.

Why do we use mole fraction?

The mole fraction defines the number of single component molecules (or moles) divided by the total number of molecules (or moles) in the mixture. When two reactive components are mixed together, the mole fraction is useful, as the ratio of the two components is understood if the mole fraction of each is known.

Is mole fraction equal to partial pressure?

In a mixture, the partial pressure of each gas is proportional to the mole fraction. The pressure exerted by each gas (its partial pressure) in the gas mixture is independent of the pressure exerted by all other gases present in the gas mixture.

What is the symbol for Avogadro’s constant?

The symbol for Avogadro’s constant is either NA or L.

Which among 4 mol of H2O and 5 mole of CH3COOCH contains the greatest number of hydrogen atoms?

Among 4 mol of H2O and 5 mole of CH3COOCH, 5 mol of CH3COOH contains the greatest number of hydrogen atoms. This is because in 4 mol of H2O there are 4N number of molecules where N is equal to Avogadro’s number. In 5 mol of CH3COOH, there are 5N number of molecules where N is equal to Avogardro’s number. Also, the number of hydrogen atoms in the formula is same as the total number of hydrogen atoms in the sample. Therefore, in 4 mol of H2O there are 8N hydrogen atoms while in 5 mol of CH3COOH there are 20N hydrogen atoms. Therefore, 5 mol of CH3COOH contains the greatest number of hydrogen atoms.

Based on the periodic table, what is the molar mass of Cs and Fe?

Based on the periodic table, the molar mass of Cs and Fe are 132.91 g/mol and 55.85 g/mol respectively.

What is 7.1 mol of Al in grams?

7.1 mol of Al is 26.98g/1mol = 191.56 grams of Al.

What is 0.134 kg of Li in moles, and what is the total number of atoms?

0.134 kg of Li is 1000g/1kg = 134 g of Li (1 mol/6.941g) = 19.3 mol of Li. The total number of atoms is 1.16 x 1025 atoms of Li.