I am taking a combined bio+A&P course and the whole molecules and atoms mess is confusing, but I’m getting it. But I can’t wrap my head around Avogadro’s number. Please help?
Edit: Thank you everyone! This was a huge help!!
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Atoms are super small, way too small for the scales we know how to make. So, if we have 10g of some element, that’s a whole bunch of atoms. But, 10g of carbon is also a lot more atoms than 10g or iron. To compensate for this, it’s necessary to name a number of atoms. Like a “dozen” is a certain number of eggs. With atoms, a dozen is still too small, so the named number is a mol. A mol of carbon atoms weighs the atomic weight of carbon (12.011 g). The number in a mol is Avogadro’s Number, like 12 in a dozen.
Avogadro’s number is just the definition of a mole. Avogadro’s number is 6.022 x 10^23 things in one mole. You can have a mole of anything. If you have a mole of bowling balls, you have 6.022 x 10^23 bowling balls. If you have a mole of pancakes, you have 6.022 x 10^23 pancakes.
Avogadro’s number is helpful in chemistry and biology because it allows for standardization based on a number of atoms or molecules. Chemistry and biology don’t really care about the weight or mass of something, it cares about the number of things you have (the amount). In the equation 2H2 + O2 –> 2H2O, there is nothing in the equation about mass. What matters is that two H2 molecules reacts with one O2 molecule to make two H2O molecules. In other words, two moles of H2 reacts with one mole of O2 to create 2 moles of H2O.
However, we can’t easily measure a mole of something. You can’t put a bunch of hydrogen under a microscope and count the number of molecules you have. What we *can* measure is mass (or weight). We can then use molar mass (the mass of one mole of something) to convert between mass and amount.
Its just a way to convert mass to number of atoms/molecules and vice versa.
If you have a chemical reaction of, for example, burning carbon C + O2 -> CO2, then you understand reaction by counting atoms in formula, one carbon atom to two oxygen atoms, easy. But if you are handling reagents, you are measuring them by mass, so you need to convert somehow.
Avagadros number of things is one mole of things. One mole of carbon atoms is 12 grams of carbon because carbon has a mass number of 12, 6 protons, and 6 neutrons.
So let’s say you are burning 12 grams of carbon. How much oxygen do you need for complete combustion?
Well, as per formula, one mole of oxygen molecules or two moles of oxygen atoms, same-same. How much is it in mass? Oxygen has 8 protons, 8 neutrons, and mass number 16, so one mole of oxygen atoms is 16 grams. You need two moles, so 32 grams of oxygen are needed.
It’s more or less the number of nucleons in 1 gram of normal matter. The weight of an atom isn’t exactly given just by the number of nucleons (because of binding energy and such), but it’s close.
So for example, roughly a gram of hydrogen contains Avogradro’s number of hydrogen atoms, or roughly 12 grams of carbon contains the same number of carbon atoms.
A mol is a number
Or have you heard?
It’s 6 times ten to the
Twenty-third…
It’s six with twenty-three
Zeros on the end
It’s much to large
To comprehend
Some numbers we give special names.
For instance, the number 12 is called a dozen. The reason 12 gets a special name it is a commonly used number and places like bakeries adopted it usage.
Similarly, Avogadro came up with a really useful number that simplifies communicating about large numbers of atoms and molecules. Instead of saying I have 100,000,000,000,000,000,000,000 molecules of water, you can say I have 0.17 mol of water. It’s just an easier way to speak and write.
It’s the same reason astronomers use light years and not feet or meters.
Yea but why is it that number?
Every element has an atomic weight (the weighted average mass of that molecule)
If you take a Mole of atoms of one type, they have a mass of their atomic weight in grams.
Hydrogen has an atomic weight of 1.00784 u, this accounts for hydrogen, duetrerium (hydrogen-2), and tritium (hydrogen-3) present in the universe. If I have 6.28×10^23 hydrogen atoms, some of those will be deuterium, some will be tritium, but the entire volume of gas as a whole will have a mass of 1.00784 g
Hydrogen-1 has an atomic mass of 1.00783 u, so if we have a Mole of hydrogen-1, it has a mass of 1.00783 g.
If we take water (2 hydrogen and 1 oxygen), oxygen has an atomic weight of 15.999 u. 1 Mole of water molecules will weigh 15.999 g + 2*1.00784 g = 17.077 g
Avagadro’s number is simply a conversion between amu (atomic mass units) and grams
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