How much does a 12 x 12 x 12 piece of aluminum weigh?

First of all, find out the volume of the piece:

V = 12 x 12 x 12

V = 1728 cm3

The density of aluminum is 2.70 g cm-3 therefore all you have to do is:

1728 x 2.7 = 4665.6 grams ---> 4.6656kg.