Transcript Unit V: The Mole Concept

Unit V: The Mole Concept
V.1. Atomic Masses and Avogadro’s
Hypothesis
John Dalton
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proposed that the atoms of each element had
a characteristic atomic weight
atoms that were the combining units in
chemical reactions
John Dalton
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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.
John Dalton
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Looked at the masses of gases
What did he find out?
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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
Joseph Gay-Lussac
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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.
Joseph Gay-Lussac
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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
Joseph Gay-Lussac
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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.
Avogadro’s Hypothesis
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Equal volumes of any gas at the same
temperature and pressure contain equal
numbers of molecules
Avogadro’s Hypothesis
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Great this explained the simple volume ratios
for gases
Problem: Who was Avogadro?