# Day 168: Reasoning with Avogadro and Gay-Lussac

As the year goes on, the more I’m struggling with teaching content vs.

teaching process. With a wealth of information available 24/7 in many

students pockets, the more I realize that the “what” is becoming less

important and the “how” is becoming more important.

And so, in chemistry, I’ve been emphasizing reasoning. In particular,

using Avogadro’s Hypothesis and Gay-Lussac’s Law of Combining Volumes

to deduce the chemical formulas of different molecules. (Thank you

Modeling chemistry!)

Avogadro’s Hypothesis:

* Equal volumes of all gases at the same temperature and pressure

contain the same number of molecules.

Gay-Lussac’s Law of Combining Volumes

* The volume of gases taking part in a chemical reaction show simple

whole number ratios to one another when those volumes are measured at

the same temperature and pressure.

In the picture, we see that 2 volumes (2 “boxes”) of hydrogen gas

react with 1 volume of oxygen gas to produce not 1, but 2 volumes of

water vapor (which we deduced previously to be H2O via the

electrolysis demo on Day 159). Therefore, hydrogen and oxygen

molecules must be diatomic. Students have found the particle/molecule

diagrams and volume boxes to be very helpful.

Further reactions for reasoning included:

1 volume nitrogen + 1 volume oxygen –> 2 volumes Gas A

What is the formula for Gas A and is nitrogen diatomic?

1 volume chlorine + 1 volume hydrogen –> 2 volumes Gas B

What is the formula for Gas B and is chlorine diatomic?

2 volumes Gas A + 1 volume oxygen –> 2 volumes Gas C

What is the formula for Gas C?

3 volumes hydrogen + 1 volume nitrogen –> 2 volumes Gas D

What is the formula for Gas D?

1 volume Gas B + 1 volume Gas D –> 1 volume Gas E

What is the formula for Gas E?

1 volume oxygen + 2 volumes carbon –> 2 volumes Gas F?

What is the formula for Gas F? Is carbon diatomic?

2 volumes Gas G –> 2 volumes Gas F + 1 volume oxygen

What is the formula for Gas G?

1 volume methane + 2 volumes oxygen –> 1 volume Gas G + 2 volumes water vapor

What is the formula for methane?