A compound decomposed by first order rxn. The concentration of compound decreases from .1180 M to .0950 M in 5.2 min. What fraction of the compound remains after 6.7 minutes?I'm thinking I set up a proportion maybe. Any ideas how to tackle this problem?

Answer 1

The problem states that it is a first order reaction so you use the first order equation of ln(No/N) = akt
Plug in 0.1180 for No and 0.0950 for N, a = 1, t is 5.2 min, solve for k.

Then ln(No/N) = kt. You know k from above, you know t is 6.7 min, solve for No/N, then take the reciprocal (to get N/No) which will be the fraction remaining. Some students get confused with this and there is another way of doing it.
You could substitute No = 0.1180, k from above and 6.7 for t and solve for N which is the concn remaining. Then solve for N/0.1180 to find the fraction remaining. Same answer either way.