Aqueous solution of ferrous sulphate is used in Mohr salt preparation. Fe2+ ion undergoes hydrolysis, hence dilute H2SO4 is added to prevent hydrolysis. 1. How does the addition of dil H2SO4 prevent hydrolysis? 2. Why cant conc H2SO4 be used?

It is quite simple.

(1) A pale green precipitate of ferrous hydroxide is formed when aqueous ammonia (NH4OH) is added to the solution of a ferrous salt.

Ferric salts give a reddish brown precipitate of ferric hydroxide.

(2) A solution of a ferrous salt acidified with dilute sulfuric acid decolourises potassium permanganate when a few drops of the purple solution of the latter are added to it.

Ferric salts do not decolourise KMnO4 as they have no reducing power.

When it comes to diluting sulfuric acid (H2SO4 with water, there is really one right (safe) way in terms of diluting what with what. The process of dilution of sulfuric acid, a very strong acid (very high dissociation of ions in the presence of water) is highly exothermic. If you put a spoon of water into a bucket of sulfuric acid, it would not be a good idea, as the reaction would be violent and give off a lot of heat. The safe way is adding, in small increments, sulfuric acid into a large amount of water. This way, the sulfuric acid only reacts in small amounts, and hence the heat given off and possible volatility of the reaction is greatly reduced and it is much safer.

In water XeF2 undergoes hydrolysis slowly. The rate of hydrolysis is accelerated in presence of alkali. Products of hydrolysis are Xe, H-F and O2.

2XeF2(s) + 2 H2O(l) → 2 Xe(g) + 4 HF(aq) + O2(g)

Hope, this helps.

Amides have a significant resonance contribution from a form that gives the nitrogen a positive charge.

This makes the amine leaving group less stable.

The resonance also tends to make the carbonyl carbon less polariseable, making nucleophilic attack harder.

Or the short answer: The amide bond is resonance stabilised compared to the ester bond.

Hydrolysis is the splitting of a compound using water. The linkage of the oxygen is split, where the ends are OH and OH. The original oxygen in the linkage bonds with one hydrogen from the water and the rest of the water, OH, attach to the other end of the molecule.

Pramit Mitra's answer to How does acidification of aqueous copper sulphate solution prevent hydrolysis?

You need to make it acid enough so it prevents hydrolyisis. You could use most any concentration of sulfuric acid, but conc. takes a bit more care in handling.