Basic Properties of Gases — Identifying Minimum Molecular Mass Among the following common gases, which one has the lowest molecular mass (kg/kmol or g/mol): Oxygen (O2), Nitrogen (N2), Hydrogen (H2), Methane (CH4)?

Introduction:
Molecular mass (molar mass) is a foundational concept that influences gas properties such as the gas constant R_specific, speed of sound, and diffusion rates. This question checks quick recall of molar masses for common gases used in thermodynamics and fluid mechanics problems.

Given Data / Assumptions:

• Diatomic molecules for oxygen and nitrogen under standard conditions (O2, N2).
• Diatomic hydrogen (H2) and methane as CH4.
• Standard atomic masses are used for estimation.

Concept / Approach:
Calculate or recall typical molar masses: H ≈ 1 g/mol, C ≈ 12 g/mol, N ≈ 14 g/mol, O ≈ 16 g/mol. Combine these according to molecular composition to obtain the molecular masses of the listed gases and identify the minimum value.

Step-by-Step Solution:
M(O2) = 2 * 16 = 32 g/mol.M(N2) = 2 * 14 = 28 g/mol.M(H2) = 2 * 1 = 2 g/mol.M(CH4) = 12 + 4 * 1 = 16 g/mol.Compare: the smallest is clearly hydrogen at 2 g/mol.

Verification / Alternative check:
Because R_specific = R_universal / M, hydrogen's very small molar mass yields the largest specific gas constant and very high molecule speeds, consistent with the identification.

Why Other Options Are Wrong:
Oxygen (32 g/mol), nitrogen (28 g/mol), and methane (16 g/mol) are all much heavier than hydrogen (2 g/mol).

Common Pitfalls:
Confusing atomic and molecular masses (e.g., taking H as 1 g/mol and forgetting the diatomic nature H2); mixing up methane's composition.

Final Answer:
Hydrogen