Question

How many hydrogen atoms are contained in 167 grams of propane?

Answer 1

Propane is C3H8

So Molar mass propane = 3*12 + 8*1 = 44 
No. of moles of propane = 167/44 = 3.795 moles

From the stoichiometry: 
No. of moles of hydrogen = 3.795 moles * (8 moles hydrogen / 1 mole propane) = 30.36 moles 

Using Avogadros number, 1 mole of any substance contains 6.022 x 10^23 atoms, therefore: 
No. of hydrogen atoms = 30.36 * 6.022 x 10^23 = 1.828 x 10^25 atoms