Molar Volume of Gas and Avogadro's Hypothesis

Experiment Objectives

• Determine the molar volume of a gas.
• Test Avogadro's hypothesis.
• Understand the concepts of limiting reagent and excess reagent in a chemical reaction.
• Determine the stoichiometric quantities of reagents required to produce a given quantity of products.
• Determine the percent yield of a chemical reaction.

Introduction to Gas Laws

One of the important points highlighted in this experiment is that one of the products of the reaction is a gas; this product will be collected to determine its molar volume. Avogadro's hypothesis states that two gases having the same volume (at the same pressure and temperature) must contain the same number of molecules. Every molecule, depending on the atoms composing it, must have a specific mass. Thus, the relative mass of a gas can be determined according to the volume it occupies. Avogadro's hypothesis allows us to determine the relative molecular mass of these gases.

Logical Steps for Molecular Mass Determination

Consider the logical order that followed:

1. The mass of 1 liter of any gas is the total mass of all the molecules within that gas.
2. One liter of any gas contains the same number of molecules as one liter of any other gas.
3. Therefore, a liter of gas has twice the mass of one liter of another gas if each molecule of the first gas weighs twice as much as the second gas molecule.
4. In general, the relative masses of the molecules of all gases can be determined by weighing equal volumes of gases.

Avogadro's Hypothesis and Relative Mass

Under normal pressure and temperature conditions (NPTC) [P = 1 atm and T = 273 K], a liter of hydrogen weighs 0.09 g and a liter of oxygen weighs 1.43 g. According to Avogadro's hypothesis, both gases have the same number of molecules. The proportion of weights between the two gases is 1.43 / 0.09 = 15.9 (approximately 16). The relationship between a molecule of oxygen and hydrogen is 16 to 1. Relative atomic masses listed in the periodic table are considered from a volume of 22.4 liters at CNPT.

Key Chemical Definitions

Gas: An aggregation state of matter in which substances have no fixed shape or volume, adopting the shape of the containers which contain them.

Ideal Gas: A hypothetical gas consisting of point particles, without attraction or repulsion between them, and whose collisions are perfectly elastic.

The Ideal Gas Law and Molar Volume

Equation I: Ideal Gas Law (PV = nRT)

The molar volume is the volume occupied by one mole of any gas. The molar volume of a gas under normal pressure and temperature is 22.4 liters, which means that one mole of any gas occupies the same volume under the same conditions of pressure and temperature.

Avogadro's Law and Gas Variables

Avogadro's Law: Equal volumes of different gaseous substances, measured under the same conditions of pressure and temperature, contain the same number of particles.

Pressure of a Gas: The force exerted by gas molecules colliding with each other and with the walls of a container, divided by the area.

Volume of a Gas: The dimensions of space occupied by a gas. In a closed system, the gas occupies the entire volume of the system.

Variables Influencing Gas Behavior

• Pressure
• Temperature
• Quantity (moles)
• Volume
• Density