The decomposition of dimethyl ether, (CH3)2O, at 510 deg C is a first-order process with a rate constant of 6.8 x 10^-4 s^-1.

(CH3)2O (g) ---> CH4(g) + H2(g) + CO(g)

If the initial pressure of (CH3)2O is 135 torr, what is its partial pressure after 1420s?

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I think you have to use

ln[(CH3)2O]t = -kt + ln[(CH3)2O]0

I tried plugging in t, k, and the initial pressure but i'm not getting 51 torr as the final answer even after I take e^x.

What am I doing wrong?