oxygen

carbon

+

+

This idea of the oxygen atom “splitting in half” violated one of the main tenants of Dalton’s atomic theory and the overwhelming evidence in support of atoms combining in whole-number ratios

How Dalton Viewed Gases

Though it’s true 2 different gases have different atomic diameters, the space that gases occupy is so large compared to their atomic size that the size of each atom has no real affect on how many can fit into same volume

How Avogadro View These Gases

+

+

oxygen

nitrogen

oxygen

hydrogen

Avogadro’s 2 key contributions:1.) equal volumes of gases contain equal number of particles2.) Some elemental gases are actually composed of more than 1 of the same type of atom (diatomic)

The importance of Avogadro’s hypothesis in determining relative mass

2 lbs

1 lb

5 lbs 4 lbs

object weight Relative weight

Basketball 4 lbs 1

baseball 5 lbs 1.2

Trying to determine relative mass this way isn’t useful or accurate• This suggests that

the weight of a baseball is 1.2x greater than the weight of a basketball

The Importance of Avogadro’s Hypothesis in Determining Relative Mass

2 lbs

1 lb

2 lbs 4 lbs

object weight Relative weight

Basketball 2 lbs 2

baseball 1 lb 1

This demonstrates that in order for us to properly determine relative mass, we need to make sure we’re talking about the same number of things if it’s truly going to be a fair and accurate comparison• This suggests that the

weight of a single basketball is 2x greater than the weight of a baseball (which is true)

Once Avogadro’s hypothesis is accepted, we can do the exact same thing we did with basketballs and baseballs……..except with atoms!!• From there, we can determine the relative mass of

certain elementsIn order to do this, we need a reference element. Something that ALL the other elements are being compared to• The mass of all the other elements are RELATIVE to that one

element we chose as a reference hydrogen

However much of an element occupied this much volume was referred to as a mole (mol)• Latin for “lump of

stuff”

22.4 L

1.99 g of hydrogen

27.91g of nitrogen

32.03 g of oxygen

Element mass Relative mass (compared to hydrogen)

Hydrogen 1.99 g 1 g/mol

Nitrogen 27.91 g 14 g/mol

Oxygen 32.03 g 16 g/mol

Though this method of determining the relative mass of elements in the gas phase produced accurate results…..• It wasn’t very reproducible considering it was insanely hard, if not impossible,

to accurately determine the density of many of the elements in the gas phase

So chemists started to use a different tool than comparing densities• Percent composition of compounds

Because oxygen is so reactive, it was easy for chemists to compare the mass of several different elements with the same amount of oxygen when they formed compounds known as oxides (XO)

For example:

100 g of CO contains 42.9 g of C and 57.1 g of O• You learned how to arrive at these numbers last unit by calculating % composition and

then figuring out the mass of each element in that compound

If we’re going to compare the mass of several different elements based on how much of each reacts with oxygen, we better use the same amount of oxygen each time to be fair

42.9𝑔𝐶57.1𝑔𝑂

=75𝑔𝐶100𝑔𝑂

We could have chose any amount of oxygen but 100 is a nice and easy number so let’s just roll with that….

Mass of different elements that combine with 100 g of oxygen

6

7

8

27.92

100

107.92

But there appears to be a slight problem with our values compared to those in the periodic

tableDalton believed that the simplest way these elements could combine with oxygen is in a 1:1 ratio• Therefore, he believed that compounds, such as water, could be represented at HO• If we accept this formula for water, we end up concluding that oxygen is only 8x

more massive than hydrogen• But here’s the problem….

Since volume ratios lead us to believe that water has 2 atoms of hydrogen for every 1 atom of oxygen, the problem with Dalton’s logic becomes obvious

We came across this problem earlier• Need to have equal number of

particles if you’re going to make an accurate comparison

12.5 200

200𝑔𝑂12.5𝑔𝐻

=16‼‼

Adjusting our previous table

6

7

8

27.92

100

107.9

12

14

16

55.8

200

215.8

The value for silver is 2x the accepted value on periodic table. This is due to the fact that it’s Ag2O

These new adjusted relative masses for each of the elements is now in agreement with what we see on the periodic table• Remember, these masses were all based on the idea of a mole • Therefore, we refer to these masses on the periodic table as

their molar mass

Molar mass: the mass of 1 mole of that particular element (g/mol)

What is the molar mass of…..

Sodium (Na):

Cobalt (Co):

Chlorine (Cl):

Neon (Ne):

We can even go as far as determine the number of actual particles in a mole!• 6.02 x 1023

How many molecules are in…..

1 mole of oxygen:

16 g of oxygen :