you decide to go on a night hike with your friend to do some stargazing making sure to bring plenty of water to stay hydrated you reach your favorite Lookout Point and Marvel at all the stars you can see your friend who is an amateur astronomy Enthusiast says can you believe how many stars there are in our galaxy I heard there's more than a hundred billion I can't imagine anything existing in Greater number always eager to flex your Chemistry knowledge you tell your friend that there are far more water molecules in your bottle of water always the skeptic your friend demands proof insisting that no one could even say for certain how many water molecules there are in there observe you say this bottle holds 500 milliliters of water since water has a density of precisely one gram per milliliter that means we have 500 grams of water now we know that the mass of one water molecule is 18 atomic mass units since oxygen weighs 16 and each hydrogen weighs one if one water molecule weighs 18 atomic mass units then by definition one mole of water molecules weighs 18 grams so it's just a matter of dividing 500 by 18 to get 27.78 as the number of moles of water molecules in this bottle now we know that one mole of anything whether it's atoms or Stars contains a number of particles equal to Avogadro's number so we can just multiply the number of moles of water molecules by Avogadro's number to get the number of water molecules in the bottle Avogadro's number is really huge 6.022 times 10 to the 23rd to be exact so if we multiply that by the 27.78 moles we get around 1.67 times 10 to the 25. that's more than 10 trillion trillion water molecules so really it's not even close in fact there are probably more water molecules in here than there are stars in the entire visible universe all right says your friend let's take the 100 billion stars in the galaxy and pretend there are water molecules how much water would we get well a hundred billion is 10 to the 11. so now we just go the other way pretending that instead of 10 to the 11 Stars we have 10 to the 11 molecules let's divide this by Avogadro's number to get the moles of water molecules we get 1.66 times 10 to the negative 12 moles multiply by 18 grams for every mole and we get 2.99 times 10 to the negative 11. that's barely more than a trillionth of a gram that's not enough to fill a bottle not enough to fill a thimble it's not even enough to see with the naked eye so don't forget when it comes to numbers stars have nothing on molecules