Chemistry Laws and Hypotheses

Law of Reciprocal Proportions

The law of reciprocal proportions states that when two elements combine separately with a fixed mass of a third element, the ratio of their masses in which they do so is either the same or a simple multiple of the ratio in which they combine with each other. This can be illustrated with the following example:

Consider the masses of hydrogen (H) and carbon (C) which combine with a fixed mass of chlorine (Cl) to form different compounds:

In HCl, 1 part of H combines with 35.5 parts of Cl.
In CCl₄, 12 parts of C combine with 142 parts of Cl.

Now, we need to compare the masses of H and C which combine with a fixed mass of Cl:

Mass ratio of H in HCl = 1:35.5
Mass ratio of C in CCl₄ = 12:142 = 1:11.83

The ratio of masses of H and C combining with the same mass of Cl is:
1/35.5 : 12/142
Simplifying this:
1/35.5 : 1/11.83 ≈ 1 : 3

Thus, the ratio of masses of H and C is a simple multiple of the ratio in which they combine with Cl.

Avogadro’s Hypothesis

Avogadro’s hypothesis provides a clear understanding of the relationship between the volume of gases and the number of molecules they contain. According to Avogadro’s hypothesis, equal volumes of all gases, at the same temperature and pressure, contain an equal number of molecules. This can be represented as:

V ∝ n

where V is the volume of the gas and n is the number of molecules.

To understand this, let’s consider the formation of hydrogen chloride (HCl) gas from hydrogen (H₂) and chlorine (Cl₂) gases under similar conditions:

H₂(g) + Cl₂(g) → 2HCl(g)

From this reaction, it can be seen that one volume of hydrogen gas combines with one volume of chlorine gas to produce two volumes of hydrogen chloride gas. Thus, it supports Avogadro’s hypothesis as the volumes of gases reacting and produced are in simple whole number ratios.

Application of Avogadro’s Hypothesis

Avogadro’s hypothesis has several significant applications:

• Molecular Weight Determination: It helps in determining the molecular weights of gases by comparing their volumes.
• Molecular Formula Determination: It aids in finding the molecular formula of gases by establishing the relation between volume and number of molecules.
• Stoichiometry of Gaseous Reactions: It simplifies the calculation of reactants and products in gaseous reactions.
• Standard Molar Volume: It defines the standard molar volume of gases, which is 22.4 liters at standard temperature and pressure (STP).

Avogadro’s hypothesis also provides a foundation for the ideal gas law, which combines various gas laws into a single equation:

PV = nRT

where P is pressure, V is volume, n is the number of moles, R is the universal gas constant, and T is temperature.