1. Unit V: The Mole Concept
V.1. Atomic Masses and Avogadro’s Hypothesis

2. John Dalton
proposed that the atoms of each element had a characteristic atomic weight
atoms that were the combining units in chemical reactions

3. John Dalton
Problem: Dalton had no method of measuring atomic weights so he made the incorrect assumption that in the most common compound between two elements, there was one atom of each.

4. John Dalton Poor John could not find a pattern
Looked at the masses of gases
What did he find out?
11.1g H2 reacted with 88.9 g of O2
46.7 g N2 reacted with 53.3 g of O2
42.9 g of C reacted with 57.1 g of O2
Poor John could not find a pattern

5. Joseph Gay-Lussac
Gay-Lussac was studying the chemical reactions of gases, and found that the ratios of volumes of the reacting gases were small integer numbers.
This provided a more logical method of assigning atomic weights.

6. Joseph Gay-Lussac Combined gas
1L of H2 reacts with 1L of Cl2 → 2 L of HCl
1L of N2 reacts with 3L of H2 → 2L of NH3
2L of CO reacts with 1L of O2 → 2L of CO2
Pattern: gases combine in simple volume ratios

7. Joseph Gay-Lussac
Problem: Dalton still equated particles with atoms, and could not accept how one particle of oxygen could yield two particles of water.
This idea was a direct threat to the relatively new atomic theory, and therefore Dalton tried to discredit the work of Gay-Lussac.

8. Avogadro’s Hypothesis
Equal volumes of any gas at the same temperature and pressure contain equal numbers of molecules

9. Avogadro’s Hypothesis
Great this explained the simple volume ratios for gases
Problem: Who was Avogadro?