Chemistry Finding rate constant from pressure change in an experiment

[HoneyBee]

Homework Statement

I measured the change in pressure for the decomposition of hydrogen peroxide in the presence of a manganese(IV) oxide catalyst at 5 different temperatures

Relevant Equations

MnO2(s)
2H2O2(l) --> O2(g)+2H2O(l)

Is there any way that I can find concentrations and then find the rate constant, k? And, using them, make an Arrhenius graph to find activation energy (including the catalyst)? Any help would be much appreciated.

Change in pressure was found using a Vernier Gas Pressure Sensor. The starting concentration of hydrogen peroxide was 3% or 0.88mol. The starting pressure was always around 98kPa. The catalyst and hydrogen peroxide were mixed together after the stopper was put on the flask. The increasing pressure was measured and the highest it went to (at 70C) was around 145kPa.
Here is an example of what I have done so far to try and find the rate constant to plot on an Arrhenius graph:

At 303.15K
PV=nRT
P/(RT)=n/v, which is the same as concentration.
120.3/(8.314*303.15) = 0.477 mol/dm^3
0.477mol/dm^3 is the final concentration of oxygen created after 180s had elapsed.
0.477mol/dm^3/180s = 0.00265 mol/dm^3/s
The ratio between oxygen and hydrogen peroxide is 1:2, therefore the rate at which hydrogen peroxide was depleted at is 0.0053 mol/dm^3/s
The ending concentration must be 0.88M - (0.0053mol/dm^3/s x 180s) = -0.074 mol/dm^3
And this is where I think something has gone wrong... The pressure continued to increase even after this 180s so I do not think the reaction went to completion.

I would then use the equation k = rate / [final concentration hydrogen peroxide] to find k.

 

[mjc123]

See my answer on chemical forums.

 

[HoneyBee]

See my answer on chemical forums.

Haha I guess you can tell I really wanted some help. Anyways thank you again!