1. The Law of Combining Volumes of Gases:
When gases react, the volumes that combine are in a ratio of small whole numbers. The ratio of the volume of each product, if a gas, is also in the ratio of small whole numbers. The volume ratio is consistent with the mole ratio from the balance chemical reaction equation.
Example:
1 Litre of hydrogen + 1 Litre of chlorine = 2 Litres of hydrogen chloride
2 Litres of hydrogen + 1 Litre of oxygen = 2 Litres of water vapour
3 Litres of hydrogen + 1 Litre of nitrogen = 2 Litres of ammonia
Mass is always conserved; but the volume of a gas is not.
Avogadro’s Hypothesis:
Equal volumes of different gases contain the same number of particles. The particles of a gas may be atoms or molecules. One liter of hydrogen = one liter of chlorine = one liter of hydrogen chloride in terms of particles (read atoms or molecules)

2. Conceptual basis of Avogadro’s Hypothesis
The particles of any gas all occupy the same space at a given temperature and pressure, independent of the composition of the gas. Thus, a given number or “particles” (atoms or molecules) of a gas will occupy the same space/volume.
One litre of hydrogen gas, contains the same number of hydrogen “particles” as the number of oxygen “particles” in one litre of oxygen gas or the number of nitrogen “particles” in one litre of nitrogen gas.
Consider the experiment:
1 L of hydrogen + 1 L of chlorine ---> 2 L of hydrogen chloride
Consider the interpretations:
H2 + Cl2 ---> 2 HCl Avogadro

3. V1/n1 = V2/n2 as long as T and P are held constant
Like Charles’ and Gay-Lussac’s laws, this relationship is in direct proportions. When there are no gas particles, there is no volume (graph goes through the origin). As gas is added, volume increases.
Therefore, V and n are in direct proportions.
V = const n or V/n = const.
V1/n1 = V2/n2 as long as T and P are held constant

4. The mass of 22.4 L of any gas at STP is the molar mass of the gas!
Amazing Consequence of Avogadro’s Hypothesis
The molar volume of a gas is the volume of one mole of the gas (at standard temperature and pressure, STP). This molar volume at STP is 22.4 L
Standard Temperature and Pressure (STP)
T = 0oC (273.15K)
P = kPa , 1.0 atm, 760 mmHg, 760 torr
V = 22.4L
The mass of 22.4 L of any gas at STP is the molar mass of the gas!

5. Alternative Standard
Standard Ambient Temperature and Pressure (SATP)
“Room conditions”
T = 25oC or K
P = 100 kPa
V = 24.8L
Use the combined gas law to convert from given T and P to standard conditions to use molar volume.

6. Homework:
Page 579 #1-3
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