4- nitrophenol is more acidic than 3,5 - dimethyl –4-nitro-phenol. Explain.

It is explained in terms of inductive effect and hyperconjugation.

Explain why aniline is less basic than ammonia?

The lone pair present at the nitrogen in aniline is delocalized in the ring (by 

resonance) and hence, it is not free for protonation, while in ammonia. It is 

present at nitrogen all the time, hence it is readily available for ptrotonation.

Why benzyl carbonium ion is more stable than ethyl carbonium ion?

Due to resonance benzyl carbonation is more stable than ethyl carbonation.

Among orthochlorophenol and orthofluorophenol, which will be a stronger acid and why?

The one having a weaker conjugate base will be a stronger acid. If the conjugate 

base has to be weak, the negative charge has to be delocalised to a larger 

extent.

In o-chlorophenol

Due to the availability of vacant orbitals in chlorine, the negative charge is delocalised to a larger extent. The same cannot take place in case of F as F does not have vacant orbitals. So, o-chlorophenol, having a weaker conjugate base, becomes a stronger acid.

Related Questions

(1)Benzylamine is a stronger base than aniline. Explain.

(2) Why is always the resonance effect dominating over the inductive effect?

(3) Why benzyl carbonium ion is more stable than ethyl carbonium ion.?

Alcohols and Phenols both are acidic but phenols are more acidic. Why?

(1)  alcohols fails to turn blue litmus red while phenol do so.

(2) alcohols fails to neutralize alkalies whereas phenols do so:

(3) Both phenol and phenoxide ions exhibit resonance, whereas alcohols and alkoxide ion do not.

(4) Due to resonance, positive charge 's developed over "O"of phenol, this weakens O-H bond and facilitates release of a proton. On the other hand. due to electron donating nature of alkyl group. -O-H bond is strengthened. therefore. it hinders the release of proton.      

(5) The phenoxide ion is more stable than phenol, due to o 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 direcrion. Due to these fact it is clear that phenols are more acidic than alcohols. 

Related Questions:

(1) Benzamide is less easily hydrolysed than methyl benzoate. Why ?

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

(3) Phenoxide ion is more stable than an alkaoxide. why?

(4) What is Steric Inhibition of Deprotonation effect ? Or SID effect ?

(5) What is the ortho effect? Why are nearly all ortho substitute benzoic acids (except some groups) stronger acid than benzoic acid?

(6) Why is o-toluic acid (2-methyl benzoic acid) stronger than benzoic acid ?

(7) Which is more acidic 2-methyl benzoic acid , 3-methyl benzoic acid or 4-methyl benzoic acid.?