Gases deviate from ideal behavior due to intermolecular forces and molecular size

Steps:

• Ideal gas law assumes negligible molecular volume and no intermolecular attractions.
• Real gases deviate more under conditions where these assumptions fail, especially with polar or larger molecules.
• Compare molecules: HCl is polar and relatively large; He is monatomic and small; CH4 and N2 are nonpolar and smaller.
• HCl has strongest dipole-dipole forces, causing greatest deviation.

Why A is correct:

• HCl is polar with significant dipole-dipole interactions, violating ideal gas assumptions of no attractions (van der Waals equation accounts for this via 'a' parameter).

Why the others are wrong:

• B. He: Monatomic noble gas with no intermolecular forces and tiny size, behaves nearly ideally.
• C. CH4: Nonpolar with weak London dispersion forces and small size, close to ideal.
• D. N2: Nonpolar diatomic with minimal forces, similar to CH4 in ideal behavior.

Final answer: A