Start by determining the percent composition of carbon and hydrogen in hexane. This can be done by using the molar masses of carbon, hydrogen, and hexane.
Essentially, you will determine the percentage carbon and hydrogen each hold in hexane by dividing each element's contribution, i.e. their molar masses multiplied by their respective number of atoms, by the molar mass of hexane, then multiplying by 100
This means that every 100 g of hexane contains 83.62 g of carbon and 16.38 g of hydrogen.
Next, you need to figure out exactly how much hydrogen you have - use Avogadro's number to go from atoms to moles and hydrogen's molar mass to go from moles to grams
This means that you'll get
You can determine the mass you're interested in without actually calculating percent composition; here's how you'd do that.
Determine the number of moles of hydrogen you get
Since 1 molecule of hexane needs 6 carbon atoms and 14 hydrogen atoms, 1 mole of hexane will need 6 moles of carbon and 14 moles of hydrogen.
Use carbon's molar mass to go from moles to grams