Answer : The concentration after 225 s is, 0.099 M
Explanation :
As we know that, the graph of ln [A] versus time yields a straight line with slope 'k'.
So, Slope = k = 
Expression for rate law for first order kinetics is given by:

where,
k = rate constant = 
t = time passed by the sample = 225 s
a = initial amount of the reactant = 0.260 M
a - x = amount left after decay process = ?
Now put all the given values in above equation, we get


Therefore, the concentration after 225 s is, 0.099 M