Calculate the mass of oxalic acid dihydrate needed to prepare 500...

Molarity of the solution is given by $M = \dfrac{n}{V}$, where $n$ is the number of moles of solute and $V$ is the volume of the solution in liters.

Given $M = 0.02 \text{ M}$ and $V = 500 \text{ mL} = 0.5 \text{ L}$.

Number of moles of oxalic acid dihydrate required is $n = 0.02 \times 0.5 = 0.01 \text{ mol}$.

Molar mass of oxalic acid dihydrate ($H_2C_2O_4 \cdot 2H_2O$) is $126 \text{ g mol}^{-1}$.

Mass required $= n \times \text{Molar mass} = 0.01 \times 126 = 1.26 \text{ g}$.

Answer: $1.26 \text{ g}$