Eli5 Moles and GFM


I have a chem test this week and I’m absolutely stumped on this, I’m terrible at math so any help would be appreciated.

In: 1

What is it about moles you are struggling with? Also what does GFM stand for? I have a degree in biochemistry and I’d be happy to help with a bit more info 🙂

A mole is just a number. A big number but its a number of particles. It’s useful for comparing amounts of substances for reactivity, because if you have a reaction where one atom of A reacts with one atom of B but a and b have different weights per atom then you can’t say 1 kg of A reacts with one kg of B but you can say one mole rescts with one mole, and then work out mass you need from the molar masses of each substance . Molar mass is also the amount that one mole weighs in grams.

Not sure what the G stands for in G formula mass though.

I’ll try my best to summarize [this video](https://youtu.be/Z_TjGRPPR9Q) by Steve Mould: say you have some chlorine and some hydrogen, and you want to make hycrochloric acid – you need the same number of atoms of each element. So how many grams of each do you put in to ensure a complete reaction? Well, you can do their atomic weight, but in grams instead of atomic weight units. And if you need more, you can do two times their atomic weight, but in grams instead of atomic weight units. “Their atomic weight, but in grams instead of atomic weight units” is a mouthful, so we call that one mole for short.

But what if your molecules aren’t elements going in? What if it’s something like hydrochloric acid with lithium hydroxide? Well, you’ll do the sum of the componants’ atomic weights, but in grams instead of atomic weight units. Once again, that’s an entire mouthull, so we just say one GFM.

It’s actually rather simple. “Mole” (M) may be called a unit, but it’s kind of not, it’s just a number. 1 M consists of roughly 6.02214076×10^(23) atoms, no matter what element.

The gram formula mass is a means to describe the mass of an atom or molecule. The unit is gram per mole (g/M), meaning the mass of 1 mole (6.02214076×10^(23) atoms) of the given substance. It is used to determine how much mass of a substance you need to get a complete chemical reaction. The GFM of elements is in the periodic table and you should have access to one during exercises and exams.

As an example: 1 M of Oxygen would completely react with 2 M of Hydrogen to become water, because each molecule of the water consists of 1 Oxygen atom and 2 Hydrogen atoms.

O + 2 H = H2O

The GFM of Hydrogen is ~1 g/M and that of Oxygen is ~16 g/M.

So you need

1 M * 16 g/M = 16 g

of Oxygen and

2 M * 1 g/M = 2 g

of Hydrogen, to have a complete reaction, with no atom being left over.

Now, the GFM of water is the combined GFM of the atoms of the molecule. It’s 2 Hydrogen atoms and 1 Oxygen atom for each water molecule, so

2 * 1 g/M + 1 * 16 g/M = 18 g/M