Half-life time of a reaction is time at which reactant concentration becomes half of its initial value.
Half-life of the first order reaction is 20 min. Rate constant can be calculated as follows:
The rate expression for first order reaction is as follows:
initial number of molecules of reactant are , time is 100 min thus, putting the values to calculate number of reactant at time 100 min,
On rearranging,
Or,
Therefore, number of molecules unreacted will be