Learn Dutch: Heb jij een bak om de pitten in te bewaren? - Do you have a container to keep the seeds in?

This is an attributive prepositional infinitive. We are describing the noun een bak (a container). The original idea is iets in een bak bewaren (to keep something in a container). When we turn this into an om ... te clause to describe that container, the preposition (in) is stranded and placed right before the te + infinitive. It forms a cluster at the end: in te bewaren.

Why is de pitten placed where it is?

In an om ... te construction, the direct object and any other extra details must go between om and the verbs at the end of the clause. Because de pitten is the direct object of the action, it is sandwiched right between om and the stranded preposition plus verb (in te bewaren).

Could I leave out in and just say een bak om de pitten te bewaren?

Leaving out in is grammatically possible but sounds incomplete, much like saying 'a container to keep the seeds' instead of 'a container to keep the seeds in' in English. The preposition connects the action back to the container: you store things inside it, so in clarifies that physical relationship.

What is the difference between pitten and zaden?