Answer
25 mL of that ibuprofen solution.
Work Step by Step
1. Write the dilution expression, and solve for the initial volume $(V_1)$
$C_1V_1 = C_2V_2$
$$V_1 = \frac{C_2V_2}{C_1} $$
2. Substitute the values and calculate the initial volume:
$$V_1 = \frac{(100. \space mL)(2.0 \space \% (m/v))}{8.0 \space \% (m/v)}$$
$$V_1 = 25 \space mL$$
