2. Gases React in Whole Number Ratios
1800s, Gay-Lussac’s law of combining volumes of gases: Observed that gases that reacted with each other did so in “whole number ratios.” He first observed that 2L of Hydrogen gas reacted with 2L of Oxygen gas to produce exactly 1L of water vapor. How does that work? Think about the reaction equation for hydrogen and oxygen.

3. Which of the following could be the correct ratio for the unbalanced reaction given below?
Once you have an answer, check it with a partner that is sitting close to you.
P4(g) + O2(g)  P2O5
2:5:2
1:2:2
1:5:2
2:5:1

4. Which of the following could be the correct ratio for the unbalanced reaction given below?
Once you have an answer, check it with a partner that is sitting close to you.
1 P4(g) + 5 O2(g)  2 P2O5
2:5:2
1:2:2
1:5:2
2:5:1

5. Hydrogen gas + Chlorine gas = hydrogen chloride gas
Which of the following supports Gay-Lussac’s law of combining volumes of gases?
Reaction:
Hydrogen gas + Chlorine gas = hydrogen chloride gas
2L + 2L = 2L
1L + 2L = 2L
3L + 2L = 1L
3L + 3L = 6L

6. Hydrogen gas + Chlorine gas = hydrogen chloride gas
Which of the following supports Gay-Lussac’s law of combining volumes of gases?
Reaction:
Hydrogen gas + Chlorine gas = hydrogen chloride gas
H2(g) + Cl2(g) --> 2 HCl
2L + 2L = 2L
1L + 2L = 2L
3L + 2L = 1L
3L + 3L = 6 L

7. Did you know?
Equal volumes of gases under the same conditions contain the equal number of molecules, no matter what type of gas they are? This is called Avogadro’s Law

8. The things people used to believe:
People believed that water was made of only one hydrogen and one oxygen, having the chemical formula OH.
John Dalton (who studied the atom) incorrectly guessed the formula for water since it “seemed the most likely formula for such a common compound.”
Avogadro came along and fixed this whole misunderstanding.

9. What do you notice? Recall: 1 mol = 6.022x1023 molecules/atoms Gas
Molar Mass
Mass of 1.0 mol of the Gas
Volume at STP Conditions
Helium
g/mol g
22.4 L
Oxygen
g/mol g
Neon
g/mol g
Recall: 1 mol = 6.022x1023 molecules/atoms

10. ONE MOLE OF ANY GAS HAS A VOLUME OF 22. 4 L AT STP CONDITIONS (101
ONE MOLE OF ANY GAS HAS A VOLUME OF L AT STP CONDITIONS ( kPa and 0.00°C)

11. WHAT IS THE VOLUME OF 0.500 MOL OF OXYGEN GAS AT STP CONDITIONS?

12. In a chemical reaction, the coefficients can indicate moles, volume, or molecules.
Why is this useful you may ask?

14. C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g)
Practice Problem
Propane, C3H8 , is a gas that is sometimes used as a fuel for cooking and heating. The complete combustion of propane occurs according to the following balanced equation.
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g)
What will be the volume, in liters, of oxygen required for the complete combustion of L of propane?
What will be the volume of carbon dioxide produced in the reaction? Assume that all volume measurements are made at the same temperature and pressure.

15. Solution

16. Practice Questions
What volume (in milliliters) at STP will be occupied by mol of methane, CH4?
What would the units for R (the ideal gas law constant) if pressure was in atm?
Nitrous oxide is sometimes used as a source of oxygen gas:
2 N2O(g)  2N2(g) + O2(g)
What volume of each product will be formed from 2.22 L of N2O?
Challenge Question: At STP, what is the density of the product gases when they are mixed?

17. Why use NOS (nitric oxide) instead of pure oxygen in vehicles?
When you compress nitrous oxide gas into a bottle it goes into a liquid – it condenses. It’s a bit like looking through a clear cigarette lighter, with its own vapor or "gas" above holding it in this state. Liquid is hugely more dense or compact than a pressurized gas can be so It takes up much less space than compressed gas does and solves its own storage problems. It also means that the bottle pressure never changes as the content level varies. Its sits there in perfect equilibrium as long as there is at least some liquid nitrous oxide left. Only the temperature can change the pressure.