Explanation:
The total pressure of a mixture of two gases is the sum of the partial pressures of each gas. This is known as Dalton's Law of Partial Pressures.

Dalton's Law of Partial Pressures:
Dalton's Law of Partial Pressures states that the total pressure exerted by a mixture of non-reacting gases is equal to the sum of the partial pressures of each individual gas in the mixture.

Partial Pressure:
The partial pressure of a gas is the pressure that it would exert if it occupied the same volume alone at the same temperature. In a mixture of gases, each gas exerts its own partial pressure independent of the other gases present.

Explanation of Option A:
Option A states that the total pressure of a mixture of two gases is the sum of the partial pressures of each gas. This is the correct answer.

When two gases are mixed together in a container, each gas exerts a pressure due to the collisions of its molecules with the walls of the container. This pressure is known as the partial pressure of the gas. The total pressure of the mixture is the sum of these partial pressures.

The partial pressure of each gas can be calculated using the ideal gas law, which states that the pressure of a gas is directly proportional to the number of moles of the gas present and the temperature, and inversely proportional to the volume of the container.

Therefore, the total pressure of the mixture is the sum of the partial pressures of each gas present. This is because each gas exerts its own pressure independent of the other gases, and the total pressure is the combined effect of all the gases present.

Conclusion:
In conclusion, the total pressure of a mixture of two gases is the sum of the partial pressures of each gas. This is known as Dalton's Law of Partial Pressures. Each gas exerts its own pressure independent of the other gases present, and the total pressure is the sum of these partial pressures.