Avogadro’s Law

(source: 자바실험실, https://javalab.org)

1. What is Avogadro’s Law?

Avogadro’s Law, proposed by Italian scientist Amedeo Avogadro in 1811, is a fundamental principle in chemistry. Simply put:

Equal volumes of all gases, at the same temperature and pressure, contain the same number of molecules.

2. The Scientific Context Before Avogadro

To understand Law, it’s crucial to know the scientific background of the time:

• 1783: Antoine Lavoisier discovered that water is composed of hydrogen and oxygen.
• 1805: Joseph Louis Gay-Lussac published his law of combining gas volumes (gases react in simple whole number ratios).

However, scientists at the time couldn’t clearly distinguish between atoms and molecules, leading to confusion.

3. Avogadro’s Discovery Process

1. Problem Identification: Avogadro recognized the need for a hypothesis to explain Gay-Lussac’s law.
2. Hypothesis Formation: He proposed that “equal volumes of gases contain equal numbers of particles.”
3. Logical Reasoning:

• If 1 volume of hydrogen reacts with 1 volume of chlorine to form 2 volumes of hydrogen chloride…
• According to Avogadro’s hypothesis, the number of particles before and after the reaction should be the same.
• Therefore, Avogadro concluded that hydrogen and chlorine molecules must consist of two atoms each.

1. Conclusion: This allowed Avogadro to clearly distinguish between the concepts of atoms and molecules.

4. Experimental Proof of Avogadro’s Law

Avogadro’s Law wasn’t immediately proven by direct experiment, but later experiments provided indirect proof:

1. Perrin’s Experiment (1909):

• Aim: Measure Avogadro’s number (particles per mole)
• Method: Observing Brownian motion of oil droplets
• Result: Approximately 6 × 10²³ particles/mole

1. Millikan’s Oil Drop Experiment (1909):

• Aim: Measure the charge of an electron
• Method: Observing the motion of charged oil droplets
• Result: Enabled calculation of Avogadro’s number

1. X-ray Crystallography (from 1912):

• Method: Analyzing crystal structures
• Result: Allowed calculation of the number of molecules per unit volume

5. Practical Application of Avogadro’s Law

Here’s a simple experiment that beginners can easily understand to verify Avogadro’s Law:

Simple Experiment: Reaction of Hydrogen and Oxygen

Materials:

• 3 glass tubes
• Hydrogen gas
• Oxygen gas
• Ignition source

Procedure:

1. Fill the first tube with 2 volumes of hydrogen gas.
2. Fill the second tube with 1 volume of oxygen gas.
3. Mix hydrogen and oxygen in the third tube and ignite.

Results:

• After the reaction, water is formed in the third tube, and all gas disappears.
• This shows that exactly 2 volumes of hydrogen reacted with 1 volume of oxygen.

Explanation:
This result can be explained by Avogadro’s Law. Since equal volumes of gases contain equal numbers of molecules, 2 molecules of hydrogen reacted precisely with 1 molecule of oxygen to form water.

6. Conclusion

While Avogadro’s Law may seem simple, it’s crucial for understanding the fundamentals of chemistry. This law helps us comprehend the invisible world of molecules and predict chemical reactions. Even beginners can grasp the importance of Avogadro’s Law through such simple experiments and explanations.

Avogadro’s Law continues to be a cornerstone of modern chemistry, playing a vital role in various calculations and predictions in chemical reactions and gas behavior.