Answer
I need $1.0 \times 10^2$ mL of this 25% (m/v) glucose 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:
$$V_1 = \frac{(500. \space mL)(5.0 \space \% (m/v))}{25 \space \% (m/v)}$$
$$V_1 = 1.0 \times 10^2 \space mL$$