1 mole of aluminium oxide, Al₂O₃ = 2  x 27 + 3 x 16 = 102 u = 102 g
102 g Al₂O₃ has = 6.023 x 10²³ Al₂O₃ molecules
0.051  g Al₂O₃ has = \(\frac { 6.023\times { 10 }^{ 23 } }{ 102 } \)
= 3.01 x 10²⁰ Al₂O₃ molecuels
1 molecule of Al₂O₃ gives = 2Al⁺⁺⁺ ions
Hence 0.051 g Al₂O₃ gives = 2 x 3.01 x 10²⁰ Al⁺⁺⁺ ions
=6.023 x 10²³ aluminium ions.

Molecular masses of H₂ = 1 +1 = 2 u
Molecular masses of O₂ = 16 +16 = 32 u
Molecular masses of Cl₂ = 35.5 + 35.5 = 71 u
Molecular masses of CO₂ = 12 + 32 = 44 u
Molecular masses of CH₄ = 12 x 2 + 1  x 6 = 30 u
Molecular masses of C₂H₆ = 12 x 2 + 1  x 4 = 28 u
Molecular masses of NH₃ = 1 x 14 + 1 x 3 = 17 u
Molecular masses of CH₃OH = 1 x 12 + 1 x 3 + 1 x 16 + 1 x 1 = 32 u

Number of atomes in 1 mole or 24 g Mg = 6.022 x 10²³
Number of atoms in 6 g Mg = \(\frac { 6 }{ 24 } \times 6.022\times { 10 }^{ 23 }\)
= 2.5 x 10²³ atoms
Now 6.022 x 10²³ number of atoms in grams of sodium = 23 g
2.5 x 10²³ number of atoms in grames of sodium = \(\frac { 23\times { 2.5\times 10 }^{ 23 } }{ 6.022\times { 10 }^{ 23 } } \)
= 9.548 x 10^-1 g.

(a) Carbon dioxide
(b) Elements: Carbon and oxygen.Valency of oxygen is -2 and carbon is +4 in CO
(c) Number of moles in 44 g CO = 1
Number of mole in 360 g CO = \(\frac { 360 }{ 44 } \) = 8.18 moles
(d) Number of molecules CO in 44 g = 6.022 x 10²³
Number of molecxules of CO in 88 g = \(\frac { 88 }{ 44 } \times 6.022\times { 10 }^{ 23 }\)=12.044 x 10²³

(a)Molar mass of nitrogen = 14 g
5.6 gh of nitrogen = \(\frac { 5.6 }{ 14 } \)= 0.4 mole
(b) 28 g nitrogen = 6.0222 x 10²³ molecules.
5.6 g of nitrogen = 2.15 x 10²² molecules
(c) 14 g of nitrogen =  6.0222 x 10²³ atoms.
5.6 g of nitrogen =  4.30 x 10²² atoms.

1 u = 1/12th of 1 atom of C-12
1 mole C-12 = 12 gram
12 gram C-12 = 6.022 x 10²³ atoms
or  6.022 x 10²³ atoms of C-12 = 12 grams
One atom of C-12 = \(\frac { 12 }{ 6.022\times { 10 }^{ 23 } } \)g
1 u = \(\frac { 12 }{ 6.022\times { 10 }^{ 23 } } \times \frac { 1 }{ 12 } \)
= 1.660302 x 10^-24 gram.

1. Metals exists as atomic crystals and do not form molecules.Generally, metals and other elements like carbon and silicon do not have single molecular structure but consists of a very large indefinite number of atoms bonded together.A form of carbon bonded with sixty atoms is called buckminsterfullerene
2. Non-metals exist as molecules containing one or more atoms.For example, argon exists as Ar and nitrogen as N₂
3. Compounds containing non-metals only exist as molecules.For example, compound of nitrogen and hydrogen is a ammonia molecule.Compound of carbon  and oxygen is a carbon dioxide molecule.
4. Compounds containing metals and non-metals exist as ions.For example, compound of sodium and chlorine consists of positivity charged sodium ions and negatively charged chloride ions. 

(i) According to Dalton's atomic theory, matter is invdivisible, i.e which cannot be divided.But now it is known that under special circumstances, atoms can be divided into still smaller particles called electrons, protons and neutrons.
(ii) Dalton's atomic theory fails to explain why substances like charcoal, graphite and diamond have different properties when all these substances are made up the same type of atoms, called carbon atoms.
(iii) Dalton's atomic theory postulated that all the atoms of the same element have exactly the same mass.It is now known that isotopes are atoms of the same element but have different masses.
(iv) It is known that some atoms called isobars have same masses but these belong to different elements.whereas Dalton's atomic theory says that atoms of different atoms have different masses.

50 g of 10% lead nitrate means the solution contains 5 g lead nitrate and 45 g water.Similarly, 50 g of 10% sodium chloride means the solution contains 5 g sodium chloride and 45 g water.
Thus total contents before reaction = 5+5+90 = 100 g
After reaction, amount of water = 90 g
Amount of precipitate= 6.83 g
Since according to law of conservation of mass, the total mass of reaction mixture = 100 g
Amount of sodium nitrate = 100-90-6.83 = 3.17g