Question

چطور contain رو در فرم مجهول بکار ببرم؟

مثلا نمک حاوی فلوراید است

Answer 1

Fluoride is contained in the salt. (or you could just say "is in/ within".)

But why do you want to use it in a passive voice?!

In my opinion, it sounds more natural to say:

The salt contains Fluoride.

or The salt includes Fluoride.

It may help: http://goo.gl/3HmM7F