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 3.16*10-2 M, the concentration of PH3 will be M after 99 s have passed

Answer: 5.32 x 10⁻³ M

Explanation:

1) The rate law for a first order reaction is:

r = - d [A] / dt = k[A]

2) When you integrate you get:

[ A] = Ao x e ^ (-kt)

Remember that here A is PH₃

3) Plug in the dа ta: Ao = 0.0316M, k = 0.0180 / s, and t = 99s

[PH₃] = 0.0316 M x e ^ ( - 0.0180/s x 99s) = 5.32 x 10⁻³ M