chemical equation...please help

[roxstar1]

In a chemical reaction, a certain compound changes into another compound at a rate proportional to the unchanged amount. If initially there is 20 grams of the original compound, and there is 16 grams after 1 hour, when will 75% of the compound be changed?


im pretty lost on this problem, so far I made an equation of dc/dt = kc (c= compound) and I dont really know what to do next. Any help would be greatly appreciated.

 

[Unco]

(c = amount of compound)

Separate variables:

\(\displaystyle \L\mbox{ \frac{1}{c} \frac{dc}{dt} = k}\)

Now integrate both sides with respect to \(\displaystyle \mbox{t}\). Don't forget to chuck an arbitrary constant in.

(Note that \(\displaystyle \L\mbox{\int \frac{1}{c} \frac{dc}{dt} dt}\) 'cancels' down to \(\displaystyle \L\mbox{\int \frac{1}{c} dc}\) by the chain rule.)