dalton, gay-lussac, and avogadro

8/12/2019 Dalton, Gay-Lussac, and Avogadro

1/18

Dalton Gay-Lussacand AvogadroContributions to Atomic Mass


2/18

Connecting Mass to Moles One of the greatest challenges early

chemists faced was trying to find a way

to connect the massof a substance tothe number of particlesin the sample.

It was determined that elementary

particles combined in fixed ratiosbyweight.


3/18

Connecting Mass to Moles This led Daltonto the atomic model of

matter

Example: The mass ratio of oxygen tohydrogen in water is 8:1

This does not tell us how manyatoms of each

element are involved

It could tell us this if we knew the relative massof

each kind of atom


4/18

Relative Mass To assign relative masses to elements it

is necessary to know that the samples

being compared have the same numberof particles

If particles are all the same size, the same

size sample would have the same number ofparticles

If particles are different sizes, the same size

sample would have different numbers of

particles


5/18

Relative Mass At the time, chemists did not know which

was true and tended to think the latter

was more likely Example: A bucket of baseballshas fewer

balls than an identical bucket of golf balls

If this is true in the macroscopic world, whywouldnt it be true in the sub-microscopic

one?


6/18

Relative Mass Consider earlier this year when we

studied density: was iron more dense

than aluminum because iron had moreparticlesper given volume than

aluminum or because irons individual

particles were more massivethanaluminums? Could it be some

combination of both?


7/18

Relative Mass The truth is, based on the experiments

we conducted earlier in the year, we

couldnt say which was true. Daltondid not know what was true during

his time either.

Since the mass of individual atoms could notbe determined, a system of atomic masses

had to be determined by comparison.


8/18

Relative Mass To determine a system of masses by

comparison, one elementwould have to

be chosen as the basis of comparison forall others

Dalton chose hydrogenand assigned it a

mass of 1.


9/18

Relative Mass To find the mass of another element like

oxygen:

Compare the masses of equal numberofoxygen and hydrogen atoms OR

Find the combining massesof oxygen and

hydrogen in water


10/18

Relative Mass Dalton thought that the former approach

was invalidbecause he thought identical

volumes of hydrogen and oxygen gaseswould have different numbers of particles

He thought the latter was validbut did not

take into account that it is valid ONLY ifthe ratio of atomic combinationis known


11/18

Reactions of Gases Research conducted by Gay-Lussac

suggested that equal volumesof gases,

at the same temperature and pressure,contain equal numbers of particles


12/18

Reactions of Gases Gay-Lussac noted that gases appear to

react in simple integer ratios

Example: Twovolumes of hydrogen reactedwith onevolume of oxygen to produce two

volumes of water

These findings appeared to contradict theidea that equal volumes of gases have

equal numbers of particles


13/18

Reactions of Gases Why? Well, if water is was H2O, then two

volumes of hydrogen and one volume of

oxygen should make onevolume ofwater

+ + =


14/18

Reactions of Gases Gay-Lussacs law that equal volumes of

gases have equal numbers of particles

and Daltons atomic theory did not seemto be compatible

In 1811,Avogadrowrote a paper that

reconciled the two


15/18

vogadros Hypothesis Avogadro assumed

Equal volumes of gases have equal

numbers of molecules These molecules can be split into half-

moleculesduring chemical reactions

That molecules of elemental gases could

contain more than a single atom


16/18

vogadros Hypothesis Two volumes of hydrogen react with one

volume of oxygen to produce two

volumes of water when hydrogen andoxygen can be splitinto half-molecules!

+ + = +


17/18

vogadros Hypothesis If we accept Avogadros Hypothesis, we

can compare the mass of various gases

and deduce the relative massof themolecules

To do this, we pick a weighable amount

of the lightest element (how about 1.0 g?)then use mass ratiosto assign atomic

massesto the other elements


18/18

Implications If two volumesof hydrogen combine with

one volumeof oxygen gas, it is

reasonable to assume that two moleculesof hydrogen are reacting with eachmolecule of oxygen

The word chosen to represent the

standard weighable amount of stuff, themole, comes from the Latin mole culaor little lump

dalton, gay-lussac, and avogadro

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