Dalton's Law of Partial Pressures at Constant Temperature

Steps:

• Calculate total initial amount of gas using PV (proportional to moles at constant T): for X, 12 kPa × 5 dm³ = 60 kPa·dm³; for Y, 6 kPa × 10 dm³ = 60 kPa·dm³.
• Add totals: 60 + 60 = 120 kPa·dm³.
• Final volume is 5 + 10 = 15 dm³.
• Final pressure = total PV / total V = 120 / 15 = 8 kPa.

Why A is correct:

• Dalton's law states total pressure equals sum of partial pressures; each partial pressure is initial P × (initial V / total V), yielding 4 kPa + 4 kPa = 8 kPa.

Why the others are wrong:

• B ignores volume difference, averaging pressures as (12 + 6)/2 = 9 kPa.
• C assumes equal volumes or misadds totals, possibly 150/15 = 10 kPa.
• D overestimates by using wrong total PV, like 165/15 = 11 kPa.

Final answer: A