The molar mass of MgCl2 is 95.211 g/mol. To calculate the number of moles in 0.684 g of MgCl2, we can use the formula:

Moles = Mass / Molar mass

Moles = 0.684 g / 95.211 g/mol

Moles = 0.00718 mol

Therefore, there are 0.00718 moles in 0.684 g of MgCl2.