Different way to find weight

[steevey]

I remember hearing something like this from a person I'm no longer in contact with, and it was something like, you can figure out the weight of substances by adding the weight of all the individual atoms in a substance, and you could also find how much atoms there are in a pure substance based on the weight, or something similar to that. Does anyone know anything about this? So say I have a block of pure iron that weighs 10 pounds. If I know the mass of an individual atom of that iron block which is like some negative 10th power kilogram or something, could I figure out the weight by multiplying the mass of that atom, by the gravitational constant on Earth, and then multiplying that by the number atoms?

[Mr Skeptic]

Yeah, it's pretty simple. All you have to do for these is cancel out the appropriate units until you end up with the unit you want. If your mass for the atoms is per atom, you can get the number of atoms in a mass by dividing mass/(mass per atom), and if your mass is for a mole of atoms then doing that would give you the number of moles of atoms, which with Avogadro's number will give you the number of atoms. It's more complicated if you have more than one type of atom, but still works if you know the ratios.

[John Cuthber]

However you figure it a ten pound lump of iron has a mass of ten pounds and a weight of about 45 Newtons.

 

You can calculate that by saying it's made up from about a hundred moles of iron atoms so that's about 100 * about 6000000000000000000000000 atoms each with a mass of a zillionth of a kilogram, but unless you know how much each atom weighs and how many of them there are you are still stuck.