how diff substances have the same amount? is there an ecuation to explain where's the logic in avogadro's num?

Answer 1

There is logic behind it, but its not covered by an equation I'm afraid.

When we started working out there were elements, which is not that long ago, we worked out pretty quickly what their relative atomic weights were. They always combined in certain proportions of weight, and using this we were able to establish atomic weights and molecular weights.

We ended up knowing, for example, that two atoms of Hydrogen reacted with one atom of oxygen to form water.

We knew that in weight there was always 8 time as much oxygen as there was hydrogen by weight. We knew that the atomic weight of hydrogen is 1 and the atomic weight of oxygen is 16. We also knew that Hydrogen existed as H2 and Oxygen as O2.

However, telling people that 8 g of oxygen reacts with 1 g of hydrogen doesn't tell them very much that helps them to picture what is actually happening. We had no idea how many actual atoms were involved. So make it simpler, we defined something called a Mole (seemed a good name) which was a fixed number of molecules. We didn'nt know exactly how many molecules, but we knew that 1 mole of O2 would weigh 16 times as much as one mole of H2.

Since they worked in grams, they said that the weight of 1 mole of Hydrogen atoms would be one gram, and therefore the weight of one mole of oxygen atoms would be 16 grams. (strictly speaking, I think they did it the other way around, but the point is the same)

Now that we had invented this quantity, the mole, we were able to say that 2 moles of H2 reacted with 1 mole of O2 to form 2 moles of H2O. Saying it this way, instead of in weights, allowed people to see exactly what was happening (Being scientists, we had to make up a big word for it, stoichiometry, If we didn't make up big words, people would start to think that science was easy, and we wouldn't want that, would we?).

As time went on, we became smarter, and we worked out ways to calculate how many atoms or molecules there were in a mole. The guy who did this was NOT called called Avogadro. You'ld have thought it was, wouldn't you?

Avogadro was the person who suggested that equal volumes of all gases at the same temperature and pressure contain the same number of molecules, and he did this before we had worked out all the atomic and molecular weights ; in fact it was this idea (called the Avogadro Principle) which helped to do just that. Avogadro would never have heard of the concept of a "mole". But since his principle had helped to work out so many of the numbers, the number of atoms or molecules in one mole was named after him.

Since the choice of 16 grammes for one mole of oxygen was made only because the relative weight of an oxygen atom was 16, we shouldn't expect the number to come out as a nice easy to remember number. And it doesn't. Sorry.

Answer 2

The logic is that avogadros number is roughly the inverse of the proton and neutron mass in grams. Thus avogadros number worth of protons (H) weighs 1gramme. The same amount of Helium weighs 4g because there are two protons and two neutrons. That's the logic of it. as far as how different substances having the same amount, that only goes for when there is an equal amount of moles of each. As the mole is defined as being a certain amount (avogadros number) the two substances will have to have the same amount (of molecules that is).

Answer 3

No. of particles = No. of moles of substance * Avogadro's number

When you look at equation, you discover that the different masses (or amounts) of substances result in the same number of particles.

Although the mass is related to the number of moles, they are NOT proportional to each other for different substances;

Moles = Mass / Relative Molecular Mass

where the relative molecular mass varies from substance to substance.

Answer 4

Take 16 grams of oxygen and 17 grams of Chlorine. Both have 1 mol of moleclues, meaning each sample has 6.022 x 10^23 number of molecules.

But because they are different substances, with different atoms, 1 mol (6.022 x 10^23 molecules) of each substance has a different mass.

Answer 5

There is just a simple logic behind it. Whenever you get to 6.022*10^23 atoms or molecules you have 1 mole of it. 1 mole is more like a count of the number and not the mass.