Number of Moles are calculate by using following formula,
Moles = Mass / M.mass --- (1)
M.mass is calculated by adding the atomic masses of all elements present in molecule, Since, MgCl₂ contains one Mg (atomic mass = 24.3 g/mol) and two Cl atoms (atomic mass = 35.45 × 2 = 70.9 g/mol), So,
Molar mass = 24.3 g/mol + 70.9 g/mol
Molar Mass of MgCl₂ = 95.2 g/mol
Now, putting values in eq. 1,
Moles = 310 g ÷ 95.2 g/mol
Moles = 3.256 moles