How many formula units make up 29.2 g of magnesium chloride (mgcl2)?

Formula units??? im assuming moles 
if its how many moles of mgcl2 in 29.2g of mgcl2
to find moles: Mass / RFM = 29.2/95 = 0.307mol

if its the number of atoms: Moles x Avogadro contestant
0.307 x 6.02x10+23 

hope that helps

The correct name for n3-ion should be azide.