The formula for the density of a substance expressed in mass and volume is rho = mass/volume or p = m/v. Rearranging the formula to isolate volume gives the formula v = m/p. To solve for the problem given, this formula must be used. This gives a solution of:
v = m/p = 250 g/ 968 g/cm^3 = 0.258 cm^3 of sodium