At high pressure, ammonia breaks down on a hot platinum surface by a zero order reaction: $2\,\text{NH}_3(g) \rightarrow \text{N}_2(g) + 3\,\text{H}_2(g)$. A rigid vessel at a fixed temperature starts with pure ammonia at $0.800\ \text{bar}$. The graph shows the total pressure in the vessel against time: it rises in a straight line and reaches $1.040\ \text{bar}$ after $120\ \text{s}$. In a second run at the same temperature, on the same surface, pure ammonia starts at $0.500\ \text{bar}$. Assume that the reaction stays zero order down to $0.250\ \text{bar}$ of ammonia. What is the half-life of ammonia in the second run?

Fig. 1: A graph of the total pressure in the vessel, in bar, against time, in seconds. A straight line rises from 0.800 bar at the start to 1.040 bar at 120 seconds; both points are marked.