To calculate the number of moles in 8.4 x 1023 formula units NaCl, we need to divide the given number of formula units by Avogadro's number (Nₐ), which is approximately 6.022 x 10^23. This calculation will convert the formula units into moles.

Number of moles of NaCl = (8.4 x 10^23 formula units) / (6.022 x 10^23 formula units/mole)

Solving for the number of moles:

Number of moles of NaCl ≈ 8.4 x 10^23 / 6.022 x 10^23

≈ 1.4 moles

Therefore, there are approximately 1.4 moles in 8.4 x 10^23 formula units of NaCl.