Gas volumes and stoichiometry at RTP

Steps:

• The reaction H₂ + Cl₂ → 2HCl shows 1 vol H₂ reacts with 1 vol Cl₂ to form 2 vol HCl.
• 10 cm³ H₂ fully reacts, consuming 10 cm³ Cl₂ and producing 20 cm³ HCl.
• Excess Cl₂ is x - 10 cm³, which remains unreacted.
• Final volume = 20 cm³ HCl + (x - 10) cm³ excess Cl₂ = 40 cm³, so x = 30 cm³.

Why C is correct:

• 30 cm³ Cl₂ provides exactly 10 cm³ excess after reaction, yielding 20 cm³ HCl for a total of 40 cm³, per Avogadro's law where equal volumes equal moles at RTP.

Why the others are wrong:

• A: Full reaction of 10 cm³ Cl₂ yields 20 cm³ HCl only, totaling 20 cm³.
• B: 20 cm³ Cl₂ leaves 10 cm³ excess, totaling 30 cm³.
• D: 40 cm³ Cl₂ leaves 30 cm³ excess, totaling 50 cm³.

Final answer: C