The slope of the graph [A]_t against time for zero order reaction is

For a zero order reaction, the integrated rate law is given by:

$[\text{A}]_t = -kt + [\text{A}]_0$

Comparing this equation with the equation of a straight line $y = mx + c$, we get $y = [\text{A}]_t$ and $x = t$.

The slope of the graph $m$ is $-k$ and the y-intercept $c$ is $[\text{A}]_0$.

Therefore, the slope of the graph of $[\text{A}]_t$ against time is $-k$.

Answer: $-k$