Question

A rock is 16.7% by mass fayalite (Fe2SiO4), 9.8% by mass forsterite (Mg2SiO4), and the remainder silicon dioxide. What is the mass percent of each element in the rock?

? = %Fe ?=%Mg ?=%Si ?=%O

Answer 1

let mass of rock be 100 g

mass of Fe2SiO4 = 16.7 g
number of moles of Fe2SiO4 = mass /molar mass
= 16.7/203.77
= 0.082 mol

mass of Fe = 2*0.082*molar mass of Fe
=2*0.082*55.85
= 9.15 g

mass of Si =0.082*molar mass of Si
=0.082*28.09
= 2.30 g

mass of O = 4*0.082*molar mass of O
=4*0.082*16
= 5.25 g

Number of moles of Mg2SiO4 = mass/molar mass
= 9.8/140.69
=0.0697 mol
mass of Mg = 2*0.0697*molarmass of Mg
= 2*0.0697*24.3
=3.39 g

mass of Si =0.0697*molar mass of Si
=0.0697*28.09
= 1.96 g

mass of O = 4*0.0697*molar mass of O
=4*0.0697*16
= 4.46 g

number of moles of SiO2 = mass/molarmass
= (100-16.7-9.8)/60.1
=1.22 mol

mass of Si =1.22*molar mass of Si
=1.22*28.09
= 34.27 g

mass of O = 2*1.22*molar mass of O
=2*1.22*16
= 39.04 g

Total mass of Fe = 9.15 g
Total mass of Mg = 3.39 g
Total mass of Si = 2.30 + 1.96 + 34.27 = 38.53 g
Total mass of O = 5.25 + 4.46 + 39.04 = 48.75 g

Since total mass is 100 g
%Fe = 9.15 %
%Mg = 3.38 %
% Si = 38.53 %
% O = 48.75 %