Question

A rock is 5.0% by mass fayolite(Fe2SiO4), 7.0% by mass forsterite(Mg2SiO4) and the remainder silicon dioxide. What is the mass percent of each element in the rock?

I have the answers but i don't know how to solve the problem.

How would you in steps solve the problem?

Answer 1

Start off by saying lets consider 100 g of rock. Therefore we have

5.0 g Fe2SiO4
7.0 g Mg2SiO4
88.0 g SiO2

lets look at the SiO2. 1 mole of this contains 28 g of Si and 32 g O.

Thus 1 mole or 60 g contains 28 g of Si and 32 g O.
Therefore 88.0 g contains

88.0 g x 28/60 Si = 41 g
88.0 g x 32/60 O = 47 g

Now repeat for the other compounds and add up the masses of each element. Because we chose 100 g to start with then the totals for each element will be the %.
Don't forget to do a check that all the masses add up to 100 g!