Learn Spanish (Spain): Cien gramos de queso, por favor. - One hundred grams of cheese, please.

You use cien for exactly 100 when it comes right before a noun (like gramos) or before a larger number (like mil). You only use ciento when you are saying the numbers 101 to 199, like ciento uno.

Do we always use de when talking about weights and measurements?

Yes! In Spanish, when you specify a quantity or portion of something, you must link the measurement and the item with de. Just like in English you say '100 grams of cheese', Spanish uses cien gramos de queso.

Why do we use por in por favor instead of para?

Por favor is a fixed expression that you just have to memorize. While por and para can both translate to 'for' depending on the rules, por is used in many common everyday phrases like this one, as well as por supuesto (of course) and por fin (finally).