what is the chemical formula of chock?

Calcium Carbonate

CaCO3

Sea shells are CaCO3 by the way. Could there be a connection?

I am assuming that you mean chalk.

I'm sorry, but it seems like there might be a misunderstanding or a typo in your question. I couldn't find any information about a substance called "chock" with a specific chemical formula. It's possible that you meant to ask about another substance or misspelled the name. If you can provide more information or clarify your question, I'll be happy to help you with the correct information.