4.5: A first order reaction has a rate constant 1.15 10−3 s−1 How long will 5 g of this reactant take to reduce to 3 g?

 From the question, we can write down the following information:

Initial amount = 5 g

Final concentration = 3 g

Rate constant = 1.15 10-3 s-1

−1
We know that for a 1st order reaction,

t=2.303KlogR0R
=2.3031.15×10-3×0.2219
=444.38 s
=444 s (approx)