Consider a certain reaction A Products with k = 2.0 × 10–2s–1. Calculate the concentration of A remaining after 100 s if the initial concentration of A is 1.0 mol L–1.

k=2.010-2s-1T=100s
A0=1.0mol-1
Since the unit of k is s-1, the given reaction is a first order reaction.
k=2.303tlogA0A
Therefore, 
2.0×10-2s-1=2.303100slog1.0A
2.0×10-2s=2.303100s(-logA)
-logA=2.0×10-2×1002.303
A=antilog-2.0×10-2×1002.303
 
= 0.135 mol L-1 (approximately)
 
Hence, the remaining concentration of A is 0.135 mol L-1
.