Why are there different rate constants?

[bluepen]

Hey,

The decomposition of A is first order, and [A] is monitored. The following data are recorded:
t / min 0 1 2 4
[A]/[M] 0.100 0.0905 0.0819 0.0670

Calculate k.

Using [A] = [A]₀e^-kt,

Calculating at t = 1
0.0905 = 0.1e^-k(1)
0.905 = e^-k
-0.0998=-k
k = 0.0998 min^-1

Calculating at t = 4
0.0670 = 0.1e^-k(4)
0.67 = e^-k(4)
k = 0.100 min^-1

How can this be!?
The question is an example from http://http://www.science.uwaterloo.ca/~cchieh/cact/c123/intratel.html"

Thanks,

 

[sjb-2812]

Experimental error in measuring the concentration? 0.1 and 0.0998 aren't that far apart.

 

[Borek]

In most experiments I would consider 0.1 and 0.0998 to be the same - 0.2% error is relatively small.