chemistry in the past

scientifico

Hello, how did chemists of the past know the weight of the elements to build up the periodic table without using new lab tecniques ?
Have they transformed all the elements in to gases and used Avogadro's law to obtain a relative atomic mass ? And how did they it was an element and not a compost ?

Thank you

Borek

They used quality balances to measure weight, and a lot of guesswork.

Some composite substances were considered elements till they were proved to be not.

scientifico

Yes but to compare atomic weight of two elements you must be sure you have the same number of particles in both elements, how did they solved this ?

DrDu

What exactly do you mean with "whithout using new lab techniques"?

scientifico

withou the possibility to calculate the number of particles in a mole

Borek

You don't need that, equivalent masses are relative.

scientifico

What do you mean?
If I take 1 g of Fe and 1g of S I can't calculate relative atomic weight because they don't contains the same quantity of atoms...

Borek

If you check how much Fe reacts with 1 g S you know ratio of their equivalent weights. This ratio is not some random number, but depends on the molar masses and valency of elements.

http://en.wikipedia.org/wiki/Equivalent_(chemistry)

Studiot

Dalton was the first to propose and publish a system of atomic weights.

Google "Dalton atomic weights" finds lots.

However he was right about some and wrong about other weights, as were those who followed immediately.

The story of how the correct values gradually unfolded is quite fascinating and well presented in the book

"Chasing the Molecule" by Julian Buckingham

DrDu

You don't need to have vapours. You can also infer the ammount of moles e.g. from freezing point depression of solutions of salts, boiling point increase etc.
As others pointed out before, a lot can be learned from studying the mass proportions in different compounds.

scientifico

What are that ?

Borek

Have you tried to google them or do you expect to be spoon feed all the time?