Dry-basis composition after condensation:\nA gas has molar composition 10% H2, 10% O2, 30% CO2, and 50% H2O (by moles). If 50% of the water condenses, what is the final mole percent of H2 on a dry basis?

Introduction / Context:
Process gas compositions are frequently reported on a “dry basis,” excluding water vapor. When moisture is partially removed by condensation, engineers must recalculate the composition of the remaining stream, which directly affects combustion calculations, emissions reporting, and downstream separations.

Given Data / Assumptions:

• Initial composition (mole %): H2 = 10, O2 = 10, CO2 = 30, H2O = 50.
• Fifty percent of the water vapor condenses and leaves; noncondensable components do not change in amount.
• Ideal-gas behavior; temperature and pressure changes do not create additional reactions.
• Dry basis means excluding water from the denominator after condensation.

Concept / Approach:
Select a convenient basis (e.g., 100 mol of feed gas). Remove 50% of the water by condensation. Then compute the new “dry gas” composition by dividing each noncondensable component’s moles by the total moles of the noncondensable gases (i.e., after excluding remaining water).

Step-by-Step Solution:
Choose a 100 mol basis: H2 = 10 mol; O2 = 10 mol; CO2 = 30 mol; H2O = 50 mol.Condense 50% of H2O: removed = 25 mol H2O; remaining H2O = 25 mol.Noncondensables remain: H2 = 10; O2 = 10; CO2 = 30.Dry basis total (exclude water): 10 + 10 + 30 = 50 mol.H2 mole fraction on dry basis = 10 / 50 = 0.20 → 20%.

Verification / Alternative check:
If one mistakenly divides by the wet total (including the remaining 25 mol H2O), the percentage would be 10/75 = 13.33%, which is not “dry basis.” The correct method excludes water entirely from the denominator when reporting dry-basis composition.

Why Other Options Are Wrong:
10%: this is the original value (wet basis), not the dry-basis value after condensation.5% and 18.18%: arise from incorrect denominators (e.g., partially excluding components) or arithmetic mistakes.

Common Pitfalls:
Confusing wet-basis and dry-basis definitions; forgetting to remove the condensed water from the total; using percent of original instead of recomputed normalized percentages.

Final Answer:
20%