Question

In gas analysis, when composition is expressed on a dry basis, it means:

• A. All gases are removed
• B. Water vapor is included
• C. Water vapor is excluded
• D. Inert gases are excluded

Hint:

Logic Tip: The term is highly literal. "Dry" means without water. Thus, a "dry basis" calculation operates under the mathematical assumption that the gas mixture is perfectly dry (zero water vapor).

Answer 1

The Correct Option is C

Concept:
In chemical engineering and environmental gas analysis (like flue gas from combustion), the composition of a gas mixture can be reported in two standard ways: on a "wet basis" or on a "dry basis." This depends on how the moisture content is treated in the final calculation.

Step 1:
Many industrial gases, particularly those from combustion processes, naturally contain a significant amount of water vapor ($H_2O$) alongside the primary gases like $CO_2$, $O_2$, and $N_2$.

Step 2:
If the gas mixture is analyzed as-is, the molar percentages of all gases, including the exact percentage of water vapor, are calculated so that they all sum to 100%. This is a wet basis analysis.

Step 3:
Water vapor can easily condense out of the gas if the temperature drops, which would constantly change the relative percentages of the remaining gases and make calculations inconsistent.

Step 4:
To establish a stable, normalized baseline, engineers mathematically (or physically, using a condenser) remove the water vapor from the total volume. The percentages of the remaining dry gases ($CO_2$, $O_2$, $N_2$) are then recalculated so that *they* sum to 100%.

Step 5:
Therefore, expressing a composition on a "dry basis" explicitly means that the water vapor fraction has been excluded from the analysis.