## Chemistry formulas for Atoms, Molecules and Chemical Arithmetic Part I

__Atomic Weight related Chemical Formulas__

__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 C

^{12}· 1 a.m.u. = 1.66×10

^{-24}g· Atomic Weight = Gram Atomic Weight (GAW)

· 1 Gram Atomic Weight (GAW) of every element contains 6.023×10

^{23}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 H

_{2}N_{2}O_{2}X_{2}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 related Chemical Formulas__

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

^{12}· Actual Weight of 1 Molecule = Molecular Weight x 1.66×10

^{-24}g**Methods of Determining Atomic Weight**

**1. ****Diffusion Method:**

Applicable only for gases.

·

Where, r

_{1}& r_{2}is rate of diffusion of gases and M_{1 }& M_{2}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__

__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 H

_{2}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 W

_{1}/ Weight of Metal Displaced W_{2}= Equivalent Weight of Metal Added E_{1}/ Equivalent Weight of Metal Displaced E_{2}**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 (W

_{1})/ Weight of ppt. obtained (W_{2}) = Equivalent Weight of Salt (E_{1})/ Equivalent Weight of Salt in ppt. (E_{2})**8. ****Conversion Method: **

· Weight of Compound A (W

_{1})/ Weight of Compound B (W_{2}) = (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