Chemistry formulas for Atoms, Molecules and Chemical Arithmetic Part I

Atomic Weight related Chemical Formulas

· Atomic Weight of an Element = Weight of an average Atom of that Element/ (1/12)x Mass of an element of C¹²

· 1 a.m.u. = 1.66x10^-24g

· Atomic Weight = Gram Atomic Weight (GAW)

· 1 Gram Atomic Weight (GAW) of every element contains 6.023x10²³ atoms of that element.

· No. of gram of an element = weight of element in gram/ Gram Atomic Weight (GAW) of that element

Methods of Determining Atomic Weight

i. Dulong and Pettits Method:

Applicable only for solid elements except Be, B, C, Si.

· Atomic Weight x Specific Heat = 6.4 (app.)

· Atomic Weight (app.) = 6.4/ Specific Heat (in Calories)

· Exact Atomic Weight = Equivalent Weight x Valency

· Valency = App. Atomic Weight / Equivalent Weight

ii. Vapour Density Method:

Applicable only for those elements whose chlorides are volatile.

· Valency of the Element = Molecular Weight of Chloride / Equivalent Weight of Chloride

· Valency of the Element = (2 x V.D. of Chloride) / (Equivalent Weight of Metal + 35.5)

Where, V.D. = Vapour Density

· Atomic Weight = Equivalent Weight of Metal x Valency

iii. Specific Heat Method:

Applicable only for Gases.

· Cp/Cv for monoatomic gases = 1.66

· Cp/Cv for diatomic gases = 1.40

· Cp/Cv for triatomic gases = 1.33

· Atomic Weight of Gaseous Element = Molecular Weight/ Atomicity

Where, Atomicity is number of atoms present in a molecule of a gaseous element. For example atomicity of Inert Gas is 1, atomicity of Ozone is 3, atomicity of H₂ N₂ O₂ X₂ is 2, and atomicity of Sulphur is 8.

iv. Volatile Chloride Formation Method:

· Atomic Weight of the Element = Equivalent Weight (Z) x Valency (x)

v. Isomorphism Method:

This method based on law of Isomorphism. According to law of Isomorphism, “Compounds having identical crystal structure have similar constitution and chemical formula”

· Atomic Weight = Equivalent Weight x Valency

· Weight of Element A that combines with certain weight of other elements/Weight of Element B that combines with the same weight of other elements = Atomic Weight of A / Atomic Weight of B

Molecular Weight related Chemical Formulas

· Molecular Weight = Weight of 1 Molecule of the Substance/ (1/12)x Weight of 1 atom of C¹²

· Actual Weight of 1 Molecule = Molecular Weight x 1.66x10^-24g

Methods of Determining Atomic Weight

1. Diffusion Method:

Applicable only for gases.

Where, r₁ & r₂ is rate of diffusion of gases and M₁& M₂ is Molecular Weight.

2. Vapour Density Method:

Applicable only for gases.

· Molecular Weight = 2 x Vapour Density

3. Victor Mayer Method:

Applicable only for volatile liquids and solids.

· Molecular Weight of a substance = 22400 ml of vapour of a substance at STP

Equivalent Weight related Chemical Formulas

· No. of Gram Equivalent Weight = Weight of the substance in gram/ Gram Equivalent Weight of the substance

· Equivalent Weight of an Element = Atomic Weight/ Valency

· Equivalent Weight of an Acid = Molecular Weight/ Basicity

· Equivalent Weight of an Base = Molecular Weight/ Acidity

· Equivalent Weight of a Salt = Formula Weight/ Total Positive or Negative Charge

· Equivalent Weight of a Reducing Agent = Formula Weight/ No. of electrons lost per molecule or Total change in Oxidation Number

· Equivalent Weight of an Oxidising Agent = Formula Weight/ No. of electrons gained per molecule or Total change in Oxidation Number

· Equivalent Weight of Radicals = Formula Weight of Radical/ No. of units of Charge

Methods of Determining Equivalent Weight

1. Hydrogen Displacement Method:

Applicable for metals which can displace or combine with hydrogen.

· Equivalent Weight of Metal = (Weight of metal x 1.008)/ Weight of Hydrogen Displaced

· Equivalent Weight of Metal = (Weight of metal x 11200)/ Volume in ml of H₂ displaced at STP

2. Oxide Formation Method:

· Equivalent Weight of Metal = (Weight of metal x 8)/ Weight of Oxygen

· Equivalent Weight of Metal = (Weight of metal x 5600)/ Volume in ml of Oxygen at STP

3. Chloride Formation Method:

· Equivalent Weight of Metal = (Weight of metal x 35.5)/ Weight of Chlorine

· Equivalent Weight of Metal = (Weight of metal x 11200)/ Volume in ml of Chlorine at STP

4. Neutralization Method:

· Equivalent Weight of Acid or Base = Weight of acid or base in gram/ (Volume of base or acid in litre required for neutralization x Normality of base or acid)

5. Metal Displacement Method:

· Weight of Metal Added W₁/ Weight of Metal Displaced W₂ = Equivalent Weight of Metal Added E₁/ Equivalent Weight of Metal Displaced E₂

6. Electrolytic Method:

· Gram Equivalent Weight = Electrochemical Equivalent x 96500

· Weight of X deposited/ Weight of Y deposited = Equivalent Weight of X/ Equivalent Weight of Y

7. Double Decomposition Method:

· Weight of Salt taken (W₁)/ Weight of ppt. obtained (W₂) = Equivalent Weight of Salt (E₁)/ Equivalent Weight of Salt in ppt. (E₂)

8. Conversion Method:

· Weight of Compound A (W₁)/ Weight of Compound B (W₂) = (Equivalent Weight of Metal + Equivalent Weight of Anion of Compound A)/ (Equivalent Weight of Metal + Equivalent Weight of Anion of Compound B)

9. Volatile Chloride Method:

· Equivalent Weight = {(2 x Vapour Density of Chloride)/ Valency} – 35.5

10. Silver Salt Method:

Applicable for organic acids

· Equivalent Weight of Acid = Molecular Weight of Acid/ Basicity

Chemistry formulas for Atoms, Molecules and Chemical Arithmetic Part I