The first-order decomposition 2N2O5(g) -> 4NO2(g) + O2(g) starts with pure N2O5 at 200 mmHg in a rigid vessel (ignore dimerisation of NO2). The total pressure is 290 mmHg after 20 min. The half-life in minutes (with ln2=0.693) is

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The first-order decomposition $2 N X_{2} O X_{5} (g) 4 NO X_{2} (g) + O X_{2} (g)$ starts with pure $N X_{2} O X_{5}$ at 200 mmHg in a rigid vessel (ignore dimerisation of $NO X_{2}$ ). The total pressure is 290 mmHg after 20 min. The half-life in minutes (with $ln 2 = 0.693$ ) is

p

N X_{2} O X_{5}

reacts,

P_{t} = (P_{0} - p) + 2 p + p /2 = P_{0} + 1.5 p

. So

p = (290 - 200) /1.5 = 60

mmHg and

p_{A} = 140

mmHg.

k = \frac{1}{20} ln \frac{200}{140} = 0.0178 mi n^{- 1}

t_{1/2} = 0.693/ k = 38.9

min.