Three reactions X (zero order), Y (first order) and Z (second order) each start with $1.00 mol L^{- 1}$ of reactant and each has a half-life of $10 min$ at this concentration. Select the correct statement(s) about the situation at $t = 20 min$ .

Answer: A, B

Y: two half-lives leave

(1/2)^{2} = 0.25

M. Z:

k [Z]_{0} = 1/ t_{1/2} = 0.10 mi n^{- 1}

, so

[Z] = 1/ (1 + 0.10 \times 20) = 0.33

M. X: zero order has

k = [X]_{0} / (2 t_{1/2}) = 0.050 M mi n^{- 1}

and is fully consumed at 20 min, not 0.25 M. Since 0.25 is less than 0.33,

[Y] > [Z]

is false.