The molar mass of Al(PO4)3 is 262.97 g/mol. To calculate the mass, we can use the following formula:

$$mass = moles × molar mass$$

Here, the number of moles is 12.96 and the molar mass is 262.97 g/mol:

$$mass = 12.96 mol × 262.97 g/mol$$

$$mass = 3413.72 g$$

Therefore, the mass of 12.96 moles of Al(PO4)3 is 3413.72 grams.