In this question, we have to find out how many moles of Hydrogen atoms are in 0.84 moles of C3H8, and in order to find out, we do by the following calculation:
We will need to use Avogadro's number, which is how many atoms there are in 1 mol of every substance, the value is 6.02*10^23 atoms
1 mol of C3H8 = 6.02*10^23 atoms
0.84 moles = x atoms
x = 5.06*10^23 atoms of C3H8
But in we have 8 Hydrogens in this substance
5.06*10^23 atoms * 8 = 4.048*10^24 atoms of Hydrogen
Now we divide this value by the avogadro's number:
4.048*10^24/6.02*10^23 = 6.7 moles of Hydrogen in 0.84 moles of C3H8