The dielectric con stant of H2O is 80. The electrostatic force of attraction between Na+ and Cl–will be.

Water is a polar colvent and have dielectric constant 80. As NaCl is a polar compound and like dissolves like so, forces of attraction between Na+ and Cl- ion will reduce to 1/80 in water.

Why The phenoxide ion is more stable than phenol ?

The phenoxide ion is more stable than phenol, due to greater dispersal of negative charge, therefore. Equilibrium get shifted to forward direction. whereas alkoxide ion is less stable than alcohols due to intensification of negative charge, so equilibrium get shifted towards backward direction. Hence phenols are more acidic than alcohols. 

Related Questions:

Benzamide is less easily hydrolysed than methyl benzoate. Why ?

Highly branched carboxylic acids are less acidic than unbranched acids. Explain why?

Phenoxide ion is more stable than an alkaoxide. why?

Which is more stable? CH3-CH2+ (Ethyl carbocation) or CH2=CH+ (Vinyl carbocation)? And why?

No Doubt, Ethyl carbocation (CH3-CH2+) is more stable than vinyl carbocation (CH2=CH+). The greater stability of CH3-CH2+ carbocation over vinyl carbocation because the positive charge on the CH2 is stabilised by the inductive effect as well as hyperconjugation  from the neighbouring carbon atom.
But in case of CH2=CH+. The two carbon atoms involved are sp2 hybridised. An sp2 hybridised carbon atom is more electronegative which increases positive charge and increases as the s-character of a hybridised carbon atom increases. As the the result inductive effect decreases by the neighbouring carbon atom. 

Why Guanidine behave as the strongest base among neutral compounds?. explaine the remarkable basicity of guanidine.

Basicity of Guanidine:

Guanidine is the strongest base among neutral compounds:

The remarkable basicity of guanidine is attributed to the fact that the positive charge on the guanidinium ion is delocalized equally over the three nitrogen atoms, as shown by

these three equivalent resonating  structures:

Basicity of nitrogen can be increased by attachment topi-donors (NH₂) group. These two pi-donating NH₂groups donate electron density to the (pi-accepting) C=NH. Hence, the guanidinium ion is a highly stable cation.

What is Guanidine and its structure , name amino acid which contains guanidine group ?

Guanidine is strongest organic nitrogenous compound with the formula HNC(NH₂)₂. Guanidine is analogue of carbonic acid. That is, the C=O group in carbonic acid is replaced by a C=NH group, and each OH is replaced by a NH₂ group.

A guanidine group also appears in larger organic molecules, including on the side chain of arginine (a basic amino acid).

It is a colourless solid that dissolves in polar solvents. It is a strong base that is used in the production of plastics and explosives. It is found in urine as a normal product of protein metabolism.