The gas-phase decomposition $A (g) B (g) + 2 C (g)$ is first order in $A$ . Starting with pure $A$ at 100 mmHg, the total pressure is 200 mmHg after $20 min$ . The total pressure after $60 min$ is

Answer: A

x

mmHg of A reacts,

p_{A} = 100 - x

and

P_{t} = 100 + 2 x

. At 20 min

x = (P_{t} - 100) /2 = 50

, so

p_{A} = 50

mmHg, half of the initial value, giving

t_{1/2} = 20 min

. At 60 min (3 half-lives)

p_{A} = 12.5

x = 87.5

, and

P_{t} = 100 + 2 \times 87.5 = 275.0 mmHg