Question

The half-life for a first-order reaction is 32 sec.What was the original concentration if,after 2.0 minuites, the concentration is 0.062 M ?

Answer 1

The half life is the time required for the concentration to drop to one-half its original value. 2 minutes is 2*60= 120 sec.

If you have 120 sec and time of half life, you can calculate number of concentration decreasing to one-half of its previous value.

120 sec / 32 sec = 3.75 and it means that you need divide original concentration on 2 3.75 times to get 0.062 M, so

[C]/2^3.75 = 0.062 M

[C]/13.45 = 0.062 M

[C] = 0.062 M*13.75

[C] = 0.8525 M