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

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

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.

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

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.

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

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.

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

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