Posted by Chris on .
What would be the concentration of dinitrogen pentoxide after 6.00 X 10(2) seconds and how long would it take for the concentration of N2O2 to decrease to 10.0% of its initial value in the following equation...
N2O5 to NO2 and O2
initial concentration is 1.65 X 102mol/L
rate constant is 4.80 X 104/s
time is 825s
According to my book, the answer is 1.24 X 102 mol/L and t= 4.80 X 10 (3)s
I have no idea how to set this problem up with the 1st order rate law...

Chemistry 
DrBob222,
This is a first order reaction (you know that from the units given for k).
ln(No/N) = kt
ln(0.0165/N) = 4.80E4*600
Solve for N.
Use the same equation. No is the same, N is 10% of that, k is the same, solve for t.
Check my work for typos. 
Chemistry 
jacfar,
A vessel containing 39.5 cm3 of helium gas at 25°C and 106 kPa was inverted and placed in cold ethanol. As the gas contracted, ethanol was forced into the vessel to maintain the same pressure of helium. If this required 17.8 cm3 of ethanol, what was the final temperature of the helium?

Chemistry 
DrBob222,
Hoe many mols He were in the container? That's PV = nRT. Solve for n. Use that n in another PV = nRT. You know P,
V = 39.5cc  17.8cc
R and solve for T.
Rememberr V must be i L, R is 0.08206 if you use P in atm or 8.314 if you use P in kPa. T must be in kelvin. 
To jacfar 
DrBob222,
You should not piggy back your question on another post. Go to the top of the page and click on "Post a New Question."

Chemistry 
Chris,
So, to solve for t would I set up the problem like this...
ln (0.00124/0.0165)=4.80 X 104 X t?