To calculate the mass of 9.40 moles of magnesium chloride (MgCl2), we need to know its molar mass. The molar mass of MgCl2 is the sum of the atomic masses of one magnesium atom (Mg) and two chlorine atoms (2Cl).

Atomic mass of Mg = 24.305 atomic mass units (amu)

Atomic mass of Cl = 35.453 amu

Molar mass of MgCl2 = (1 × 24.305 amu) + (2 × 35.453 amu) = 95.211 amu

Now, we can calculate the mass of 9.40 moles of MgCl2 using the formula:

Mass = Number of moles × Molar mass

Mass = 9.40 moles × 95.211 g/mol = 894.98 grams

Therefore, the mass of 9.40 moles of magnesium chloride (MgCl2) is approximately 894.98 grams.