Atoms and Molecules (Part – 3)

Chemical Formula

* It is a symbolic representation of compound.

* The combining capacity of element is called Valency.

* Example Ammonium, Hydroxide, Nitrate, Hydrogen carbonate, Sulphite etc.

* For a stable compound the ion must be balanced.

* In a compound if metals or non-metals are together then the symbol of metal is writing first and the symbol of non-metal is writing second.

* Example :- NaCl, CaO, FeS, CuO In all these Na, Ca, Fe, Cu, are metals so write first and Cl, O, S, are non-metals so write second.

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Formula of simple compound

1. Formula of sodium chloride

In this compound the valency of Na and Cl is 1

So the formula becomes NaCl

2. Formula of hydrogen chloride

In this compound the valency of H and Cl is 1

So the formula becomes HCl

3. Formula of hydrogen sulphide

In this compound the valency of H is 1 and valency of S is 2

So the formula becomes H2S

4. Formula of calcium oxide

In this compound the valency of Ca is 2 and valency of O is 2

So the formula becomes CaO

Molecular Mass

It is the sum of masses of all the atoms in molecule.

Formula Unit Mass

* It is the sum of masses of all the atoms in molecule.

* Formula Unit Mass is calculated in the same manner.

* Only difference is in this use formula.

* For example NaCl

1*23 + 1*35.5 = 58.5 u

* For water

In water hydrogen have valency 2 and oxygen 1

So 2*1 + 1*16 = 2 + 16 = 18

Mole concept

* If we see a example that two mole of hydrogen combined with one mole of oxygen and forms two mole of water.

* 4 u of hydrogen molecules combined with 32 u of oxygen and forms 36 u of water.

* In this more convenient for number of atoms rather than their masses.

* So new unit mole is used now.

* 1 mole = 6.022 * 10^(23)

* It is known as Avogadro Number.

* Mass of one mole of substance is fixed.

* 1 u hydrogen = 1 atom

* 1 u oxygen = 1 mole atoms

Milan Tomic

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