Hey guys. I’m struggling to understand the concept of moles, and was hoping someone could explain it a lot easier than in previous posts. I understand that a mole of something means that there is 6.022 x 10\^23 of that something (similar to the idea of 1 dozen = 12 things), but I don’t quite understand when for example 1 mole of Nitrogen is 14g.
If 1 mole of nitrogen means that there is 6.022 x 10\^23 nitrogen atoms, how does 1 mole of nitrogen equal 14g? Is it saying that 6.022 x 10\^23 nitrogen atoms (1 mole of nitrogen) is equal to 14g, since the mass of a nitrogen atom (single nitrogen atom) would be super small, and so we use moles to convert it into a reasonable mass for easier calculations e.g. 14g?
Hope that wasn’t too confusing :S
Thanks everyone! 🙂
In: Chemistry
>since the mass of a nitrogen atom (single nitrogen atom) would be super small, and so we use moles to convert it into a reasonable mass for easier calculations e.g. 14g?
Kind of the other way around. We use mass as an approximation for 1 mole of a particular element, because counting 6.022 × 10^(23) would be incredibly time consuming and very, *very* expensive.
Think of it like this. Imagine you sell bulk hardware wholesale to hardware retailers. A customer has placed an order for two hundred fifty thousand (250k) M10 hex nuts. They want the nuts delivered in boxes of 1,000.
As the seller, you have to decide how you’ll go about this. Counting out 250 boxes of 1,000 nuts per box would take a ridiculous amount of labor if you had people do it. You could invest in expensive machinery that automatically counts the nuts, but there is a much cheaper way.
You count out 1,000 nuts one time, then you weigh them. Since all the nuts weigh pretty much the same, 1,000 nuts should weigh the same every time.
So you count out 1,000 nuts, weigh them, and you find that the weight is 11.5 kg. Now you can simply package up the nuts at 11.5 kg per box and you know that you’ll be pretty darn close to 250k nuts when you’re done.
When working with moles of atoms, we do the same thing. We know that 6.022 × 10^(23) nitrogen atoms weighs around 14g, so if we need 1 mole of nitrogen, we just weigh out 14g and call it close enough.
But why would we need a mole of something instead of simply saying we need 14g of something? Sometimes working by quantity is easier to understand than working by mass. For example, if you want to make the molecule H2O, you need two hydrogen and one oxygen. You know the *quantity* of each element required, not the mass. You could calculate the mass, but it’s easier to just work with the quantities directly until you’re done with the math, then convert to mass at the end. Just like when someone orders 250k M10 nuts instead of ordering 2,875 kg of M10 nuts. Sometimes we need to work in quantities, not mass. That’s where the mole is handy.
To add some background here, an interesting question is why and how we know the number in detail.
Imagine you do chemistry without knowing Avogadros number. You can figure out the relative weights of different atoms/molecules, and calculate with that, and do that for the entire periodic table. Perhaps define the lightest one as 1 unit of atomic world mass.
But all your recipes will be in one measure of this, two measures of that. How can you find out how many of the tiny things there is in one gram?
There are ofc strategies, Google the early methods, it is an interesting read. But a ton of chemistry was done without knowing avogadros number.
Watch this https://youtu.be/Z_TjGRPPR9Q?si=HRHz9qyHyRQ8FStm Steve mould explains it in a very easy and intuitive way IMO
This is exactly what it means.
Moles are just a way to not have absurdly high numbers during calculations
Atoms are small. Atoms are really very very small indeed. For any useful quantity of a substance, you need a very large number of atoms to do anything useful. When we started out with chemistry, we didn’t have a good way to measure how small or how much mass a single atom had. To get around the problem of dealing with such tiny things, we came up with the idea of a mole.
A mole is simply a specific large number of a thing. You can have a mole of people, or a mole of cars. Even a [mole of moles](https://what-if.xkcd.com/4/). By careful study of chemistry, we could work out how different elements interact, and so work out how much a mole of each type of element weighs, and other properties about them. In time, as our ability to measure things got better, we became able to actually put a specific numerical value to what a mole is, and got 6.022×10^23 (Avogardro’s number).
Basically a mole is “enough atoms that I have a useful amount of the stuff”.
The reason we use moles is because atoms stay the same when they form molecules, but they form in whole number ratios. So take water, H2O. So two hydrogen atoms (with an average mass around 1 atomic mass unit each) and one oxygen atom (with an average mass around 16 atomic mass units). So the oxygen is heavier than the hydrogen, but regardless one atom will bond with two whole atoms. So when we scale it up to a reasonable size, there are over trillions of atoms involved, but the ratio is still 1:2. So we use 6^e23 as a constant, and say that amount of atoms of any element is now represented as 1. Now they’re sorta in gram ranges, that’s it. So one mole of something, is also 6^e23 individual atoms of that thing. To make water, you need 2 moles of hydrogen, for every 1 mole of oxygen, to not have a remainder. Its just easier to write and think about than 6^e23 : 2(6^e23).
Because the atoms are not all the same weight, we need more constants, the average atomic masses of atoms, to convert the unit moles to mass in grams.
Short story, mole is just a unit. Like a dozen. You can count anything in moles. 1 apple, 10 apples, a dozen apples, a mole of apples. If you wanted to know what an apple or mass of apples weighed, you just need the average mass of an apple. 1 apple time the mass of one apple, 10 apples times the mass of one apple, a dozen apples times the mass of one apple, a mole of apples times the mass of one apple.
The number at the bottom of a element period box is the atomic mass.
1 mol of element = that elements atomic mass.
1 mol of compound = sum of the atomic masses of its elements.
>Is it saying that 6.022 x 10^23 nitrogen atoms (1 mole of nitrogen) is equal to 14g, since the mass of a nitrogen atom (single nitrogen atom) would be super small, and so we use moles to convert it into a reasonable mass for easier calculations e.g. 14g?
Yes.
Nitrogen has an atomic weight of 14 (7 protons, 7 neutrons) in it’s most common isotope.
6.022 x 10^23 nitrogen atoms weighs 14 grams.
If you had some hydrogen, with an atomic weight of 1 (1 proton no neutrons), 6.022 x 10^23 hydrogen atoms would weigh 1 gram.
Why do you need to know this?
Well, let’s imagine you have 1kg of hydrogen you want to burn with oxygen. How much oxygen do you need? Well you need 2 moles of hydrogen for every mole of oxygen (because you need two hydrogen atoms for every atom of oxygen).
You have 1000/1=1000 moles of hydrogen, so you need 500 moles of oxygen. Oxygen has an atomic weight of 16 so you need 500×16=8000 grams, or 8kg.
But what if we wanted to burn an isotope of hydrogen; tritium. 1 proton, two neutrons, atomic weight 3.
You have 1000/3=333.3 moles of tritium, so you need 166.7 moles of oxygen. 166.7×16=2666.4, or 2.666kg.
1 mole is the amount of amu (atomic mass units) in a gram.
Nitrogen has a mass number of 14.0067u, so one mole of nitrogen atoms has a mass of exactly 14.0067g
Nitrogen is a diatomic molecule, so we really find it as N2, so a mole of N2 has a mass of 28.0134g
You’re right that we use it because a single atom or molecule is too small to work with, but for practical experiments, not for calculations. Or math works at all scales, but we can’t physically work at such small scales.
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