This is the chemical formula for talc Mg3(Si2O5)2(OH)2(the main ingredient in talcum powder):
An analytical chemist has determined by measurements that there are 0.055 moles of magnesium in a sample of talc. How many moles of oxygen are in the sample?

0.022 mol O

Explanation:

Mg3(Si2O5)2(OH)2

We can see that 1 mol of this substance has 3 mol of Mg.

Oxygen altogether  is 5*2 (from (Si2O5)2) + 2(from(OH)2) = 10 +2 = 12

So, 1 mol of this substance has 12 mol oxygen.

So,  1 mol of this substance contains 3 mol Mg and  12 mol O, or

ratio Mg : O = 3 : 12 = 1 : 4

1 mol Mg 4 mol O

0.055 mol Mg ---x mol O

x = 0.055*4/1 = 0.220 mol O

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