# 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. = 1.66×10-24g
·         Atomic Weight = Gram Atomic Weight (GAW)
·          1 Gram Atomic Weight (GAW) of every element contains 6.023×1023 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 1.66×10-24g

### Methods of Determining Atomic Weight

#### 1.              Diffusion Method:

Applicable only for gases.
·
Where,  r1& r2 is rate of diffusion of gases and M1 & M2is 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