To find the relationship between concentration and time, we need to use the integrated rate laws. For a first order reaction, the equation is

ln ([A]t/[A]o) = - kt

Where [A] is the amount of the substance at time t and time zero. k is the rate constant and t is the elapsed time. For concentration, we are worried about the ratio between the two species so the amount can be in a variety of units as long as they are both in the same units.

ln (15/25) = - k(2 hr)

Since we want k to be in inverse minutes, it's easiest to change the time to minutes before we calculate the value of k

ln (15/25) = -k (120 min)

k = 0.00426 min^-1

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