For gas mixtures used in chemical engineering calculations, the measured volume percent (% v/v) corresponds to which composition basis? Select the most accurate statement and assume ideal-gas behavior is the usual simplifying context.

Introduction / Context:
Engineers frequently report compositions of gas mixtures as volume percent, mole percent, or mass percent. Understanding when “volume percent” can be equated to another basis is essential for material balances, reactor design, and environmental reporting. This question focuses on the special relationship between volume fraction and mole fraction in the ideal-gas limit.

Given Data / Assumptions:

• Mixture of gases at uniform temperature and pressure.
• Ideal-gas approximation unless otherwise stated.
• Volume percent means partial volume divided by total volume, expressed as a percentage.

Concept / Approach:
For ideal gases, the equation pV = nRT shows that, at the same T and total P, a component’s partial pressure is proportional to its number of moles, and likewise its partial volume is proportional to its mole count. Therefore, under ideal-gas behavior, volume fraction equals mole fraction. For real gases with significant non-ideality, compressibility factors differ among components, and the equality does not strictly hold.

Step-by-Step Solution:

Verification / Alternative check:
Dalton’s law of partial pressures for ideal mixtures gives y_i = p_i / P = V_i / V_total = n_i / n_total, confirming the equivalence only when ideal assumptions apply.

Why Other Options Are Wrong:

• Weight percent: depends on molecular weights; not generally equal to volume percent.
• Mole percent (unqualified): true only for ideal gases, not always.
• Weight percent only for ideal gases: incorrect relationship.
• Neither under any conditions: false; equivalence does occur for ideal gases.

Common Pitfalls:
Assuming volume percent equals mole percent for high-pressure real gases without checking non-ideality; this can introduce significant errors in design calculations.

Final Answer:
mole percent only for ideal gases