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Chapter 5 – Mole Concept and Stoichiometry

The following Topics and Sub-Topics are covered in this chapter and are available on MSVgo:

Introduction

A mole is a scientific unit for measuring large quantities of small atoms and molecules. The mole is the amount of substance of a system that contains 6.02214076 ×10²³ specified elementary entities. For example, one mole of oxygen has a mass of 31.998 g and contains about 6.022 ×10²³ molecules of oxygen. 

The Avogadro Constant is defined as the number of atoms or molecules per mole of a substance. It is equal to 6.022 ×10²³, and it is expressed as NA.

  • It is the ratio of the average mass of the atom to the unified atomic mass unit.
  • A Unified atomic mass unit is defined as 1/12 times the mass of the carbon-12 atom. 
Carbon scale: Relative atomic mass(RAM) = Mass of 1 atom of an element/1/12th of the mass of one C-12 atom.

Hydrogen scale: Mass of one atom of an element/Mass of one atom of Hydrogen

Gram Atomic Mass: It is the atomic mass of an element expressed in grams.

Relative Molecular Mass (Molecular weight)

Hydrogen scale: Mass of one molecule of a substance/mass of one atom of hydrogen

Carbon scale: The relative molecular mass of an element represents how many times one molecule of the substance is heavier than 1/12th of the mass of the carbon-12 atom.

 

Gram Molecular Mass: Molecular mass of a substance expressed in grams. Gram Molecular Mass of all gases contains 6.022 ×10²³ number of molecules at S.T.P.

Gram Molecular Volume: Gram Molecular Mass/Mass per litre of gas at S.T.P. One mole of a gas occupies 22.4 lit at S.T.P.

A Mole of an element = Mass of the element/Atomic Mass or Gram atomic mass
Moles of a compound = Mass of the compound/Molecular Mass or Gram molecular mass
Mass of one atom = Atomic Mass or Gram atomic mass/6.022 ×10²³ 
Mass of one molecule = Molecular Mass or Gram molecular mass/6.022 ×10²³ 
Number of atoms or molecules = Moles × 6.022 ×10²³ 

The law states that equal volumes of gases contain an equal number of molecules under identical temperature and pressure conditions.    

      V ∝ n, n is the number of moles.

                V =K n

For example, A molecule of N₂ contains two atoms of Nitrogen, and a molecule of NH₃ contains three atoms of hydrogen.

  1. Gay-Lussac’s law of combining volumes
    The law states that when gases combine at constant temperature and pressure, the volume of gases in a chemical reaction is present in the ratio of simple whole numbers. This law is valid only for gases.

Hydrogen Chloride

When one volume of hydrogen is reacted with one volume of chlorine at the same temperature and pressure, it gives two volumes of hydrogen chloride gas. The ratio of the reaction is 1:1:2

H₂ +Cl₂ → 2HCl

 

  1. To determine the relationship between molecular mass and vapour density.
Relative Vapour Density = Mass of molecules of a gas or vapour/Mass of molecules of hydrogen measured at standard temperature and pressure.

According to Avogadro’s law, the volumes at standard temperature and pressure conditions may be substituted by molecules. So,

Relative vapour density = Mass of 1 molecule of gas or vapour/Mass of 2 atoms of Hydrogen

 

Relative molecular mass = Mass of 1 molecule of a substance/ Mass of 1 atom of Hydrogen

Vapour Density = Relative Molecular Mass/2

So, it becomes 

Relative Molecular mass = 2 × Vapour Density
1. Mass of the solute /Mass of the solute+solvent × 100

2. Total weight of an element in one molecule/Gram Molecular weight of the compound  × 100

It is the ratio of atoms of different elements present in one molecule of a compound. The empirical formula of Glucose is CH₂O.

Determination of Empirical Formula

  • Firstly, write down the percentage weight and atomic weight of the elements present in a compound.

                       Percentage masses of Zn = 47.8 and Cl = 52.2 

                       Atomic masses of Zn= 65 and Cl =35.5

  • Now, divide the percentage weight by the atomic weight to get the number of atoms present in the compound’s molecule.

                        Number of atoms of Zinc = 47.8/65 =0.73 and chlorine = 52.2/35 = 1.46

  • The ratio of atoms of different elements is obtained by dividing it by a relatively small number.

                        Simplest ratio of Zinc is 0.73/0.73 =1 and chlorine = 1.46/0.73 =2

  • Write the empirical formula by writing the symbols of elements with several atoms.

                         The empirical formula of the compound is ZnCl₂.

The chemical formula denotes the actual number of atoms of different elements present in the compound’s one molecule.

Molecular formula = (Empirical formula)ₙ

Example: Molecular Formula of the Blue Vitriol is CuSO₄.5H₂O

Stoichiometry is relatively essential for understanding the relationship between reactants and products of chemical reactions. The reactions are balanced in terms of moles. A balanced chemical reaction gives equivalences in moles that allow stoichiometry calculations to be performed. The mole concept is a convenient method of expressing the amount of a substance.

  1. How do you perform mole-to-volume conversions?

 

  1. What are the limitations of Avogadro’s law?
  • The law works only for ideal gases.
  • Avogadro stated that for an ideal gas, 1 mole of particles would occupy a specific volume at a particular ratio of pressure and temperature. So it does not apply to a liquid or solid.
  • The deviation of real gas from ideal gases increases at low temperatures and high pressure.

 

  1. What are the applications of the mole?
  • You know that the mass of a mole of a substance is called a molar mass. 
  • Molar mass is used to convert grams into moles. If the mass of the substance is known, you can calculate the number of moles of a substance.
  • The mole concept is applied to the composition of chemical compounds.

 

  1. What are the mole fraction and mole percent?
    Mole fraction is defined as the number of moles of one component divided by the total number of molecules in a mixture. Multiplying the mole fraction with 100 gives the mole percent.

 

  1. What is the difference between density and vapour density?
    Density is the ratio of the mass of the substance to its volume. Vapour density is the density of gas concerning the density of hydrogen.

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High School Physics

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