how do scientists know theres trillions of molecules in water

Question

I randomly had a thought in my head and now im curious, how do scientists know that atleast trillions of molecules in let’s say a water bottle is an accurate number? Because its says that it is a factual statement but how did they get to the point where they knew it was actually atleast in the trillions Thanks.

Edit: thank you so much for the answers guys!! 🙂

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Answer 1

There are ways to figure out *exactly* how much a single molecule weighs.

So then you just weigh the entire bottle, divide by the weight of a single molecule, and you get the amount of molecules.