The reaction A -> B + C

is 1st order w/ respet to A. It's rate const k is equal to 0.343 M/s at 273K. The initial concentration of A was 0.399 M. Calculate the concentration of B after 0.185 secs have passed.

Use ln(No/N) = kt.

No = 0.399
N = ?
k = 0.343
t = 0.185 sec
This will give you (A) after 0.185 sec. Since there is 1 mole B for each mole A, B should be the same.