To what voltage is the capacitor charged, if its charge is 0.5 mC, and the energy is 2.5 mJ.

Q = 0.5 mC = 0.5 * 10 ^ -3 Cl.
W = 2.5 mJ = 2.5 * 10 ^ -3 J.
U -?
The energy of a charged capacitor W is determined by the formula: W = Q * U / 2, where Q is the charge on the capacitor plates, U is the voltage between the capacitor plates.
Let us express the voltage value from the formula: U = 2 * W / Q.
U = 2 * 2.5 * 10 ^ -3 J / 0.5 * 10 ^ -3 C = 10 V.
Answer: the voltage between the plates of the capacitor is U = 10 V.