Avogadro's law: equal moles mean equal molecules

Steps:

• Molar mass of Cl₂ is 71 g/mol, so moles in 71 g = 71/71 = 1 mol.
• Molar mass of H₂ is 2 g/mol, so moles in 2 g = 2/2 = 1 mol.
• Number of molecules = moles × Avogadro's number (6.022 × 10²³).
• Ratio of molecules (Cl₂ : H₂) = 1 mol : 1 mol = 1:1.

Why A is correct:

• Avogadro's law states equal volumes (or moles at STP) of gases contain equal numbers of molecules; here, both are 1 mol.

Why the others are wrong:

• B: Reverses the equal mole ratio.
• C: Assumes chlorine has twice the molecules, ignoring molar mass adjustment.
• D: Compares raw masses, not moles or molecules.

Final answer: A