For a first order reaction, the concentration of reactant at time t is given by
[R]t=[R]0e−k1t
The amount of reactant decomposed is
[R]0−[R]t=[R]0−[R]0e−k1t=[R]0(1−e−k1t)
The fraction of molecules decomposed is
[R]0[R]0−[R]t=[R]0[R]0(1−e−k1t)=1−e−k1t
Answer: 1−e−k1t