The gas phase decomposition of phosphine at 120 °C PH3(g)1/4 P4(g) + 3/2 H2(g)

is first order in PH3 with a rate constant of 1.80×10-2 s-1.

If the initial concentration of PH3 is 8.48×10-2 M, the concentration of PH3 will be 2.45×10-2 M after
s have passed.