27.0 grams
Assuming the block is a cube, it's volume will be 2.5^3 cm^3 which is
2.5 cm * 2.5 cm * 2.5 cm = 15.625 cm^3
Looking up the density of magnesium, its density is 1.738 g/cm^3, so multiply the volume by the density giving
15.625 cm^3 * 1.728 g/cm^3 = 27.000 g
