Concept of atomic, molecular and equivalent masses-class notes PDF

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Subject :- Chemistry Topic :- Concept of Atomic, Molecular and Equivalent Masses Atomic mass :The average relative mass of an atom of an element as compared to

1 12

th the

mass of one atom of carbon-12 (C-12). In other words atomic mass is a number which expresses as to how many times an atom of the element is heavier than 1/12th of the mass of carbon atom. Therefore, Atomic mass = (Mass of an atom of the element) 1 th mass of one carbon atom (C−12) 2

Atomic mass is expressed in atomic mass unit (amu or u). Atomic mass unit (u) is defined as

1 12

th the mass of carbon atom C-12.

Average Atomic Mass :It has been found that majority of the elements found to possess several isotopes. Isotopes are the different atoms of the same element possessing different atomic masses but same atomic number. The atomic mass of each isotope is determined separately and then combined according to the ratio of their proportion of occurrence. This is known as average atomic mass. If an element have three isotopes with atomic masses 𝑚1 , 𝑚2 and 𝑚3 and

these occur in the ratio of 𝑥, 𝑦 and 𝑧 respectively, then its average atomic mass =

(𝑚1 ×𝑥+𝑚2 ×𝑦+𝑚3 ×𝑧) 𝑥+𝑦+𝑧

e.g. Boron occurs in nature in the form of two isotopes B¹°, B¹¹ with atomic mass 10 and 11 in the ratio of 1 : 4 respectively. Therefore, average atomic mass of boron =

(10×1+11×4) 4+1

=

(10+44) 5

=

54 5

= 10.8

Gram Atomic Mass :The atomic mass of an element expressed in gram is the gram atomic mass or it is also called gram atom, e.g. the atomic mass of oxygen is 16 u, therefore gram atomic mass of oxygen is 16g.

Molecular Mass :The sum of the atomic masses of all the atoms in a molecule of the substance is called molecular mass of the substance. It is expressed in atomic mass unit (u). e.g. Molecular mass of H2 SO4 = 2 × atomic mass of hydrogen + atomic mass of sulphur + 4 × atomic mass of oxygen = 2 × 1 + 32 + 4 × 16 = 98.

Gram Molecular Mass :Molecular mass of a substance expressed in gram is called gram molecular mass. It is also called as gram molecule, e.g. molecular mass of N2 is 14 and its gram molecular mass is 14 g.

Equivalent Mass or Equivalent Weight :The number of parts of a substance that combines with or displaces, directly or indirectly, 1.008 parts by mass of hydrogen or 35.5 parts by mass of chlorine or 8 parts by mass of oxygen is called the equivalent mass of the substance. 1) Eq. wt. of metal = or

=

or

=

Mass of metal Mass of hydrogen displaced Mass of metal Mass of oxygen combined Mass of metal

× 1.008

× 8.0

Mass of chlorine combined

× 35.5 1

e.g. In H2 O, NH3 and CH4 one mole hydrogen combines with mole 2

1

1

oxygen, mole nitrogen and mole carbon respectively. Hence, 4 3

1

Equivalent weight of oxygen = × 16 = 8.0 2

1

Equivalent weight of nitrogen = × 14 = 4.67 3

1

Equivalent weight of carbon = × 12 = 3 4

Relation between atomic weights, equivalent weight and valency Atomic weight = equivalent weight x valency 2) Equivalent weight of acid =

Molecular weight of acid Basicity (number of replacable H+ )

e.g. Equivalent weight of H2 SO4 =

98 2

= 49

3) Equivalent weight of base =

Molecular weight of base Acidity (number of replaceable OH− )

e.g. Equivalent weight of NaOH = Equivalent weight of Ca(OH)2 = 4) Eq. wt. of salt =

40

1 74

= 40

= 37

2 Molecular weight of salt

Total positive valency of metal atoms 58.5

e.g. Equivalent weight of NaCl =

1

e.g. Equivalent weight of Na2 CO3 =

= 58.5

106 2

= 53

5) Equivalent weight of a substance that undergoes oxidation/reduction =

Molecular weight Change in oxidation number

e.g. When KMnO4 reacts under acidic conditions, change in oxidation number (From +7 to +2) is 5, hence; Equivalent weight of KMnO4 in acidic medium =

158 5

= 31.6

Vapour Density :Vapour density of a gas is the ratio of the mass of a certain volume of the gas to the mass of equal volume of hydrogen measured under same conditions of temperature and pressure. Molecular weight = 2 × vapour density Atomic weight = 2 ×

vapour density atomicity

Mole :A mole is the amount of substance that contains as many as particles that are present in 12g of carbon-12 isotope. There are 6.023×10²³ atoms in 12 g carbon-12 Number of mole = = =

Weight of substance in grams Gram molecular weight

Number of patrticles or atoms Avogadro′ s number Volume of gas in litres at NTP 22.4L

Gram Molar Volume :The volume occupied by 1mole of a substance is called the gram molar volume of the substance. The gram molar volume of perfect gas is 22.4 L at STP or NTP. STP or NTP means 273.15 K (0°C) temperature and 1 bar (10⁵ pascal) pressure. e.g. volume of 1 mole of H2 at STP = 22.4 L 1

22.4

4

4

volume of mole of H2 =

= 5.6 L

Chemical Equation :A chemical equation is the symbolic representation of an actual chemical change, e.g. Zn + H2 SO4 → ZnSO4 + H2 ↑

A chemical equation represents: • Compounds taking part in the reaction. • Products or compounds formed during the reaction. • Catalysts used for the reaction (if any).

Balancing of a Chemical Equation :When the number of atoms of an element present on right hand side [RHS] of the equation becomes equal to that of present on left hand side [LHS] of the equation is said to be balanced. To balance a chemical equation, following steps are followed: • Check the number of atoms of each element present on RHS and LHS of a equation whether they are equivalent or not. • If not, multiply the element by a number to the corresponding element. • Continue multiplying unit all numbers become equivalent to the corresponding elements. e.g.

Na2 O + HCl → NaCl + H2 O

Balancing of Equation Element LHS Na 2 O 1 H 1x2=2 Cl 1x2=2 The balanced chemical equation is Na2 O + 2HCl → 2NaCl + H2 O

RHS 1x2=2 1 2 1x2=2

Equation Based Problem :Suppose, the equation is 2H2 + O2 → 2H2 O 2moles of H2 react with 1 mole of O2 gives 2 moles of H2 O. 2H2 + O2 → 2H2 O 2 molecules of H2 react with 1 molecule of O2 gives 2 molecules of H2 O. 2H2 + O2 → 2H2 O 4 g of H2 reacts with 32 g of O2 gives 36 g of H2 O.

Limiting Reagent :The substance that is completely consumed in a reaction is called limiting reagent because it determines the amount of product. The other reactant present in excess reagent e.g. 2H2 (g) + O2 (g) → 2H2 O(g)

Strength of a Solution :It is generally expressed in terms of molarity and normality. (i)

Molarity (𝑴) It is the number of moles of solute present in 1L of the solution. It is denoted by 𝑀. Molarity (𝑀) =

Number of moles of solute Volume of solution (in L)

e.g. 0.25 mol L⁻¹ (or 0.25 M) solution of NaOH means that 0.25 mol of NaOH is dissolved in 1 L of solution.

(ii)

Formality (𝑭) It is the number of gram formula weight of solute dissolved in 1 litre of solution when formula weight equals to the atomic weight, then formality equals to molarity. Formaliy (𝐹) =

(iii)

Gram formula weight of solute Volume in litre

Normality (𝑵) It is the number of gram equivalents of solute dissolved per litre of the solution. Normality (𝑁) =

(iv)

Gram equivalent of solute Volume of solution(L)

e.g. 0.50 g equiv.L⁻¹ (or 0.50 N) solution of H2 SO4 means that 0.50 g equiv. of H2 SO4 is dissolved in 1 litre of solution. Molality (𝒎) It is the number of moles of the solute dissolved in 1 kg of solvent. It is denoted by 𝑚. Molality (𝑚) =

Number of moles of solute Mass of solute (in kg)

e.g. 1.00 m kg⁻¹ (or 1.00 m) solution of KCl means that 1 mol (74.5 g) KCl is dissolved in 1 kg of water.

Molality is independent of temperature whereas molarity, normality and formality change with temperature. This is because volume depends upon temperature and the mass do not....