Basic Concepts of Mole, Avogadro’s Number and Molar Volume

Explanation

We know that atom is an exceptionally little particle. The mass of an individual atom is an extremely small quantity. It is not possible to weigh individual atoms or even a small number of atoms directly. That is why we use the atomic mass unit(AMU) to express the atomic masses. For the sake of benefit, the atomic mass might be given up any unit of measurement i.e. grams, kg, pounds, and so on.

When the compound at our disposal is an element then the atomic mass of that component expressed in grams is called one gram atom. It is likewise called one-gram mole or just a mole of that element.

Number of gram atoms= Mass of an element in grams/

 or moles of a componentMolar mass of a component

For example

1-gram atom of hydrogen = 1.008 g.

1-gram atom of carbon = 12.000 g.

and 1-gram atom of uranium = 238.0 g

It suggests that one gram atom of different components have different masses in them. One mole of carbon is 12 g, while 1 mole of magnesium is 24g. It likewise shows that an individual atom of magnesium is two times as heavy as an atom of carbon. The molecular mass of a compound expressed in grams is called gram molecule or gram mole or simply the mole of a substance.

Variety of gram particles or moles of a molecular substance = Mass of molecular substance in grams/ Molar mass of the substance.

For instance.

1gram molecule of water = 18.0 g.

1gram particle of H₂SO₄ = 98.0 g.

and 1gram molecule of sucrose = 342.0 g

It indicates that one gram molecule of different molecular compounds has different masses. The formula unit mass of an ionic substance expressed in grams is called the gram formula of the compound. Given that ionic compounds do not exist in molecular type therefore the sum of atomic masses of individual ions gives the formula mass. The gram formula is likewise described as gram mole or simply a mole.

Number of gram formulas or moles of a substance = Mass of the ionic substance in grams/ Solution mass of the ionic substance

1gram formula of NaCl = 58.50 g.

1gram formula of Na₂CO₃ = 106 g.

1gram formula of AgNO₃ = 170g.

It might also be discussed here that the ionic mass of an ionic type expressed in grams is called one gram ion or one mole of ions.

For instance:

1 g ion of OH- = 17g.

1 g ion of SO42-= 96g.

1 g ion of CO3 2- =60g.

So, the atomic mass, molecular mass, formula mass, or ionic mass of the compound expressed in gram is called the molar mass of the substance.

Therefore, the quantitative meaning of mole is the atomic mass, molecular mass, or formula mass of a compound expressed in grams is called a mole.

Avogadro’s Number

Definition:

Avogadro’s number is the number of atoms, molecules, and ions in one gram atom of an element, one gram molecule of a compound, and one gram ion of a substance, respectively.

Representation: It is represented by the symbol ‘NA ‘.

Description

It is interesting to know that different masses of elements have the same number of atoms. Hence, the 6.02 ×l0²³number of atoms, molecules, or A formula units is called Avogadro’s number that is equivalent to one ‘mole’ of the respective compound. In basic words, 6.02 ×l0²³particles amount to one mole as twelve eggs are equal to one dozen.

To understand the relationship between Avogadro’s number and the mole of a substance let us consider a few examples.

• i. 6.02 ×l0²³atoms of carbon are equivalent to one mole of carbon.
• ii. 6.02 ×l0²³molecules of water are equivalent to one mole of water.
• iii. 6.02 ×l0²³formula units of NaCl are equivalent to one mole of sodium chloride.

Thus, 6.02 ×l0²³atoms of elements or 6.02 ×l0²³particles of molecular substance or 6.02 ×l0²³units of ionic substances are equivalent to 1 mole.

For further explanation about the number of atoms in molecular substances or the number of ions in ionic substances let us discuss two examples:

1. One particle of water is comprised of 2 atoms of hydrogen and 1 atom of oxygen, hence 2×6.02 ×l0²³atoms of hydrogen and 6.02 ×l0²³atoms of oxygen make up one mole of water.
2. One formula unit of sodium chloride includes one sodium ion and one chloride ion. So, there are 6.02 ×l0²³variety of Na⁺ ions and 6.02 ×l0²³CI^– ions in one mole of sodium chloride. Therefore, the overall number of ions in 1 mole of NaCl is 12.04× l0²³ or 1.204× 10²⁴.

Molar Volume

One mole of any gas at standard temperature and pressure (STP) occupies a volume of 22.414 dm³. This volume of 22.414 dm³ is called molar volume and it holds true only when the gas is ideal.

Ideal gas: An ideal gas is defined as one in which all collisions in between atoms or molecules are perfectly elastic and in which there are no intermolecular attractive forces. One can say it as a collection of completely hard spheres which collide but which otherwise do not interact with each other.

With the help of this information, we can convert the mass of a gas at STP into its volume and vice versa.

For this reason, we can state that.

• 2.016 g of H₂ = 1 mole of H2 = 6.02 x 10²³ molecules of H₂ = 22.414 dm³ of H₂ at S.T.P.
• 16g of CH₄ = 1 mole of CH4 = 6.02 x 10²³ particles of CH₄= 22.414 dm³ of CH₄ at S.T.P.

It is very intriguing to understand from the above information that 22.414 dm³ of each gas has a different mass however the very same number of particles. The reason is that the masses and the sizes of the molecules don’t affect the volumes. Generally, it is understood that in the gaseous state the distance between molecules is 300 times greater than their diameters.

 

Multiple-Choice Questions (MCQs) with Answers:

 

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Problems/Solutions Based On Mole Concept