To calculate the number of formula units in 8.56 x 10-3 mol of NaCl, we can use the Avogadro's number, which is 6.022 x 1023 formula units per mole.

Number of formula units = Number of moles * Avogadro's number

Number of formula units = (8.56 x 10-3 mol) * (6.022 x 1023 formula units/mol)

Number of formula units = 5.16 x 1021 formula units

Therefore, there are 5.16 x 1021 formula units present in 8.56 x 10-3 mol of NaCl.