Why Is My Calculation of the Initial Rate of Decomposition for NO2 Not Accepted?

[chhhnhhsh]

Homework Statement

What is the Initial Rate of Decomposition for NO₂.
Where 2NO₂-----> 2NO + O2
K=.54
Concentration at t=0 is .008 M
This is a second order reaction.

Homework Equations

Rate=k[C]²

The Attempt at a Solution

Rate=.54[.008]²=.000035 (directed to give a two sig fig answer)

This answer is not being accepted, I'm not sure what I'm missing?

 

[tiny-tim]

welcome to pf!

hi chhhnhhsh! welcome to pf!

isn't it 5.4, not .54 ?

 

[chhhnhhsh]

Thank you for the welcome!
Sorry I misprinted the information above.
A previous question for this problem was "what is the value of the rate constant" and the text calculates it for you as k=(.54 / M*s).

What is really getting to me is a previous question as well where it asked for the instantaneous rate at t=100 where the concentration = .00559 M.

To calculate this I did the same thing.
rate=.54(.00559)² = .000017 M/s. The program accepted this answer but said I should have rounded to .000018 M/s. Which I didn't understand because the real answer is .000016874.
Should I be using a different rate calculation?

 

[epenguin]

Maybe it's a bit of a trick question based oon the fact that the rate constant is a 2nd order one for bimolecular reaction 2NO₂ -> ..., but they ask for "rate of decomposition of NO"₂. ?