A sample of propane, C3H8, contains 17.7moles of carbon atoms. How many total moles of atoms does the sample c?

According to the formula of propane, C₃H₈, 1 mole of C₃H₈ molecules contain 3 moles of carbon atom and 8 moles of hydrogen atoms.

Hence,
(No. of carbon atoms) : (Total number of atoms) = 3 : (3 + 8) = 3 : 11

Total number of atoms = (17.7 mol) × (11/3) = 64.9 mol

1 mole of propane, C3H8, from the chemical formula contains 3 moles of carbon and 8 moles of hydrogen or 11 moles of atoms per mole of propane.

Total atoms = 11 moles of atoms / 3 moles of carbon atoms * 17.7 moles carbon atoms

You're told how many moles of carbon atoms you have. Moles of H atoms = 17.7 moles carbon atoms * 8 moles H atoms / 3 moles C atoms.