$$C_1V_1 = C_2V_2$$

where:

- \(C_1\) is the initial concentration \((\text{2.81 M})\)

- \(V_1\) is the initial volume \((\text{unknown})\)

- \(C_2\) is the final concentration \((\text{0.8 M})\)

- \(V_2\) is the final volume \((\text{150 mL or } 0.15 \text{ L})\)

Solving for \(V_1\), we get:

$$V_1 = \frac{C_2V_2}{C_1}$$

Substituting the values, we get:

$$V_1 = \frac{(0.800 \text{ M})(0.150 \text{ L})}{2.81 \text{ M}}$$

$$V_1 = 0.0409 \text{ L}$$

$$V_1 = \boxed{40.9 \text{ mL}}$$