Explanation:
21. The given molecule for cracking is tetradecane.
On cracking it forms one mole of decane (C10H22) and two moles of ethene gas.
The chemical equation is shown below:

22. The essential condition for the formation of an ester is the reaction of alcohol and acid in presence of concentrated sulfuric acid.
Thus among the given options, the first option is the correct one.
23. Isomers of butanol are shown below:
It is 2-butanol.
The position of -OH group changes to the second carbon.
A molecule of hydrogen is formed by two hydrogen atoms, that is a fact.
How does it work? When two atoms, known as "diatomic" pair with another in a bond known non-polar covalent bonds. Where they equally share electrons. A Hydrogen atoms needs 1 more electrons to fill its first shell fully and have a full valence shell. So if two H's share their electrons, they'll both have a full V-Shell!
That's the basics of both the H-H bond and all the other diatomic bonds as well.
There are
4.517
⋅
10
23
atoms of Zn in 0.750 mols of Zn.