In the first order reaction, A → Products, the initial concentration of A is 1.56 M and 0.869 M after 48.0 min

Question

....What will be [A] after 1.50 h?

the equation is Ln(A/A0) = -KT according to my book....

Answer 1

You need to use the equation ln (Ao/A) = kt to determine k from the first "set" of data, then figure out what A will be at 90 minutes

k = ln (Ao/A) / t = ln (1.56/0.869) / 48 min = 0.546 / 48 = 0.0114 min ^-1

A at 90 minutes = Ao / e^kt = 1.56 / e^(0.0114)(90)
A = 1.56 / 2.790 = 0.559 M

Answer 2

First order reactions have the solution
A= Ao exp(-k*t)
First you substitute in A= 0.869, Ao=1.56 and t=48 min and solve for k. Once you have k, you solve for A for t=90 min.
Roughly, expect about 40% remaining.