''Equal volume of gases under similar conditions of pressure and temperature possess equal number of molecules''. This law is given by

Options

A. Avogadro
B. Berzelius
C. Gay Lussac
D. Graham

Explanation

To solve the question "Equal volume of gases under similar conditions of pressure and temperature possess equal number of molecules. This law is given by", we will follow these steps:

### Step 1: Understand the Statement
The statement describes a fundamental principle in gas behavior, specifically concerning the relationship between volume, pressure, temperature, and the number of molecules in gases.

### Step 2: Identify the Key Terms
- **Equal volume of gases**: This indicates that we are comparing two different gases that occupy the same volume.
- **Similar conditions of pressure and temperature**: This means that both gases are under the same environmental conditions, which is crucial for the law to hold true.

### Step 3: Recall Relevant Laws
We need to recall the gas laws that relate to the number of molecules, volume, pressure, and temperature. The key law that addresses this relationship is Avogadro's Law.

### Step 4: State Avogadro's Law
Avogadro's Law states that "equal volumes of gases, at the same temperature and pressure, contain an equal number of molecules." This directly aligns with the statement given in the question.

### Step 5: Identify the Contributor
Recognizing that Avogadro's Law is attributed to Amedeo Avogadro, we can conclude that the law in question is indeed given by him.

### Step 6: Eliminate Other Options
- **Berzelius**: Known for the law of constant proportions, not relevant here.
- **Gay-Lussac**: Known for the relationship between temperature and pressure, not relevant to the number of molecules.
- **Graham**: Known for Graham's law of effusion, which is unrelated to the number of molecules in equal volumes of gases.

### Conclusion
The law stating that equal volumes of gases under similar conditions of pressure and temperature possess equal number of molecules is given by **Avogadro**.

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