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Class 10 Class 12
Iron (II) oxide has a cubic structure and each of the unit cell is 5.0 A°. If density of the oxide is 4.0 g cm-3, calculate the number of Fe2+ and O2– ions present in each unit cell.

Volume of the unit cell = (5 A°)
= (5 x 10–8 cm)3
= 125 x 10–24 cm3
= 1.25 x 10–22 cm3
Density of FeO = 4.0 g cm–3

Therefore, mass of the unit cell
= 1.25 x 10–22 cm3 x 4 g cm–3
= 5 x 10–22g
Mass of all molecules of FeO

Number of FeO molecules/unit cell

Thus, there are 4 Fe
2+ ions and 4O2– ions in each unit cell.

Why are solids rigid?

Solids are rigid because the internuclear distance between the  molecules of a solid are very less and closely packed and their positions are fixed due to the strong forces of attraction between them.

Refractive index of a solid is observed to have the same value along all directions. Comment on the nature of this solid. Would it show clevage property?

Refractive index of a solid which have the same value along all directions are isotropic in nature. It would not show cleavage property.

Classify the following as amorphous or crystalline solids: Polyurethane, naphthalene, benzoic acid, teflon, potassium nitrate, cellophane, polyvinyl chloride, fibre glass, copper.

Amorphous solids: Polyurethane, teflon, cellophane, polyvinyal chloride, fibre glass.
Crystalline solids: naphthlene, benzoic acid, potassium nitrate, copper.

Why is glass considred a super-cooled liquid?

Glass is amorphous solids and  have a tendency to flow, though very slowly. Therefore, these are called pseudo solids or super cooled liquids .

Why do solids have a definite volume?

A solid have a definite volume because the inter molecular distance  between its molecules are fixed.

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