Correct the following statements :

Graphite has a two-dimensional sheet-like structure in which each carbon atom is sp³ hybridized.

Correct the following statements :

For water-system, the number of phases that can exist in equilibrium is two.

13. Calculate the mole fraction of water in a sodium hydroxide solution which has 80 g of NaOH and 54 g of H₂O. [Relative atomic masses : Na = 23, O = 16, H = 1].

Radium has a half-life of 1580 years. How long would it take for each of the samples of radium given below to disintegrate ?

(i) 20% of a 5 g sample.

(ii) 20% of a 50 g sample.

For the cell 

Zn | Zn²⁺ (a = 1) | | Cu²⁺ (a = 1) | Cu

Given that E_Zn . Zn²⁺ = +0.761 volt; E_cu²⁺, Cu = +0.339 volt

(i) Write the cell reaction

(ii) Calculate the emf and free energy change at 298 K involved in the cell. [Faraday’s constant F = 96500 coulomb equivalent^{– 1}].

(i) What is the physical significance of entropy ?    

(ii) How is the entropy of the universe changing ?

For the extraction of aluminium, write  

(i) The most important ore.

(ii) One method for the purification of the mentioned ore.

(iii) The role of cryolite and calcium fluoride in electrolysis.