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. ?

 

[DeeNos]

Your calculation for the initial rate of decomposition is correct. However, it is possible that the answer is not being accepted because it is not given with the correct units. The units for the initial rate of decomposition in this case would be M/s or mol/L*s. Double check to make sure you have included the correct units in your answer. Additionally, make sure you are using the correct value for the rate constant (k) in your calculation. If you are still having trouble, it may be helpful to show your work or consult with a classmate or teacher for further assistance.