What does below chemistry rebus means?

How many letters are in the alphabet?

What has to be broken before you can use it?

If there are fifteen crows on a fence and the farmer shoots a third of them, how many crows are left?

I possess a head

I flaunt a tail

But I don't have mouth or legs

Can you find out who I am?

John was drowned by he was not wet. How?

You have a roll of cloth 1km long. You have a machine which cuts this roll into pieces of 10-meter-long cloth.

How long would it take for the machine to cut the roll if each cut took 4 secs?

Sherlock Holmes was challenged to make a pre-drawn line smaller without erasing it. He did it in fractions of seconds. How?

John went to buy some expensive, foreign chocolates. He only had Rs 100 with him. When he reached the shop, he got out and know that on those chocolates, there was a 15% import duty and 5% VAT.

How much worth chocolate should he buy so that he can accommodate it in Rs 100?

You may enter, but you may not come in. I have space, but no room. I have keys, but open no lock. What am I?