A melt in the binary system MgO–SiO2contains 89.92wt% SiO2. If all the magnesium is consumed to form enstatite (MgSiO3), how many moles of this mineral will crystallize from 100grams of the melt?

MgO-SiO2 contains 100-89.92 = 10.08% of MgO melt or 10.08 gm in 100 gm melt.
moles of Mg = Moles of MgO
moles of Mg = 10.08/Molecular mass of MgO
moles of Mg = 10.08/40.304 = 0.250
Moles of olivine = Moles of Mg (1:1 stoichiometry)
Moles of olivine mineral = 0.250