Note:
1805 – Gay-Lussac’s experimental Data (constant temperature and
pressure)
remember that back then they didn’t know about protons/electrons :.
No chemical formulas were yet known
1. hydrogent gas
+
oxygen gas 
20 L
10L
What is the ratio of the volumes?
Volume Ratio 2:1:2
water vapour
20 L

ammonia gas
10L
2. nitrogen gas
5L
+
hydrogen gas
15L
Volume Ratio 1:3:2
Gay-Lussac’s Law of Combining Gas Volumes
Gases react in simple whole number ratios when all gases are
measured at the same temperature and pressure.
This new law caused a major debate between Gay-Lussac and
Dalton because these experimental results did not match Dalton’s
theory
Avogadro came up with an explanation to explain why Gay-Lussac’s
experimental data was valid
Consider two identical cans (show them 2 coffee cans) :
One holding Oxygen gas
One holding Nitrogen gas
- both at the same temperature and pressure
Q: What do you think would be true about the number of molecules in
each container? It would be the same.
Avogadro’s Hypothesis
Equal volumes of gas at the same temperature and pressure contain
the same number of molecules.
Q: If the number of molecules increases at constant temperature and
pressure, will the volume increase or decrease? Increase
Q: Does this mean that volume is directly or inversely proportional to
the number of molecules?
:. Volume varies directly with # moles
remember that it is easier to count moles than molecules
The question is “what is the volume of one mole of gas?”
However volume is dependent on temperature and pressure so we
must state the temperature and pressure we are working with
We will now use the experimental results to calculate the volume of
one mole of gas at STP
Experiment
1. mass Mg = ? set up ratio where what we know is on LHS, wanted
is on RHS
0.85 g
100 cm
= mass Mg
4.4 cm
mass Mg = 3.7 * 10-2 g
2. # mol Mg = ?
how do I find # mol? # mol = mass / molar mass
# mol Mg = 3.7 * 10-2 g
24.31 g/mol
= 1.5 * 10-3 mol
3. # mol H2 = # mol Mg this is a given for now, at a later point in the
course we will learn why this is the case
# mol H2 = 1.5 * 10-3 mol
4. PH2 = Patm – PH2O
= 100.3 kPa – 2.64kPa
= 97.7 kPa
5. P1V1 = P2V2
T1
T2
(97.7 kPa) (42.3 ml) = (101.3 kPa) V2
295 K
273 K
V2 = 37.8 ml
6. 0.0378 L
= V______
-3
1.5 * 10 mol 1.00 mol
V = 25 L
Accepted Value
The volume of one mole of gas at STP is 22.4 L.
The results in our demonstration were a little off due to experimental
error
This information is needed for tomorrow to develop our last gas law
Homework: gas laws worksheet