k for a zero-order reaction is 2 × 10^–2 mol L^–1 s^–1. If the concentration of the reactant after 25 s is 0.5 M, the initial concentration must have been:

1.0 M

The correct answer is option 4. 1.0 M.

Given,

$k= 2 × 10^{−2}\text{mol L}^{1} s^{−1}$

$t= 25 sec$

Final concentration, $[A] = 0.5 M$

Since it is a zero-order reaction, so we know

$k = \frac{1}{t}\left([A]_0 − [A]\right)$

$kt = [A]_0  − [A] $

$⇒ 2 × 10^{−2} × 25 = [A]_0 − 0.5$

$⇒ 0.5 = [A]_0 − 0.5$

$[A]_0 =1.0 M$