Ubstance A undergoes a first order reaction A®B with a half-life, t½, of 20 min at 25 °C. If the initial concentration of A in a sample is 1.6 M, what will be the concentration of A after 80 min?

Answer : The concentration of A after 80 min is, 0.100 M

Explanation :

Half-life = 20 min

First we have to calculate the rate constant, we use the formula :

Expression for rate law for first order kinetics is given by:

where,

k = rate constant  =

t = time passed by the sample  = 80 min

a = initial amount of the reactant  = 1.6 M

a - x = amount left after decay process = ?

Now put all the given values in above equation, we get

Therefore, the concentration of A after 80 min is, 0.100 M