The concentration of A will be 0.34 mol·L⁻¹ after 60 min.
In a first-order reaction, the formula for the amount remaining after n half-lives is
[tex]\text{[A]} = \frac{\text{[A]}_{0}}{2^{n}}\\[/tex]
If [tex]t_{\frac{1}{2}} = \text{20 min}\\[/tex]
[tex]n = \frac{t}{t_{\frac{1}{2}}} = \frac{\text{60 min}}{\text{20 min}}= \text{3.0}[/tex]
∴ [tex]\text{[A]} = \frac{\text{2.7 mol/L}}{2^{3.0}} = \frac{\text{2.7 mol/L}}{8.0} = \textbf{0.34 mol/L}[/tex]
