Atoms and Molecules

Molecular Mass and Mole Concept

Molecular Mass

The molecular mass of a substance is the relative mass of its molecule as compared with the mass of a ¹²C atom taken as 12-units. It indicates the number of times; one molecule of the substance is heavier than

atom.

Calculation of Molecular Mass

Molecular mass is equal to sum of the atomic masses of all atoms present in one molecule of the substance.Example: H₂OMass of H atom = 1Mass of 2H atoms = 2Mass of O atom = 16Molecular mass = 2 + 16 = 18g

Relative Molecular Mass

Relative molecular mass or RMM is the molecular weight of an element or a compound. It is the number of times one molecule of the substance is heavier than 1/12 the mass of an atom of carbon (¹²C).

Gram Molecular Mass

Molecular mass expressed in grams is numerically equal to gram molecular mass of the substance.Molecular mass of O₂ = 32Gram molecular mass of O₂ = 32gGram molecular mass is the relative molecular mass expressed in grams.Note that relative atomic mass is a ratio and has no units while gram molecular mass and gram atomic mass are expressed in grams.

Mole Concept

Since it is not possible to calculate the weight of particles individually, a collection of such particles called mole is taken for all practical purposes. It was discovered that the number of atoms present in 12g of carbon of ¹²C isotope is 6.023 x 10²³ atoms. This is referred to as Avogadro number after the discoverer Avogadro. A mole of a gas is the amount of a substance containing 6.023 x 10²³ particles. It is a basic unit of the amount or quantity of a substance. The substance may be atoms, molecules, ions or group of ions.

Avogadro discovered that under standard conditions of temperature and pressure, (1 atm and 273 K) a sample of gas occupies a volume of 22.4 L.

Molar Volume

One mole of any gas at STP will have a volume of 22.4 L. This is called molar volume.

The molar volume [22.4 L at STP] plays a vital role in stoichiometric calculations because it is the link between volume and mass in reactions involving gases.

Relationship Between Gram Molecular Weight and Gram Molecular Volume

Gram molecular weight (GMW) or mole is the relative molecular mass of a substance expressed in grams. It is also called gram molecular weight of that element.

Gram molecular volume (GMV) or molar volume is the volume occupied by one-gram molecular weight of a gas at STP (Standard temperature and pressure).

All gases of equal volumes contain same number of molecules under the same conditions of temperature and pressure. Equal number of molecules of different gases will occupy equal volumes under the same conditions of temperature and pressure.

One mole of gas = 6.023 x 10²³ molecules

1 mole of a gas = 22.4 L at STP