Answer :
• In this case we are given that a molar mass of the compound is 129g/mol.
• We will use the % composition of the compound to determine how many grams of each element can be found in this compound with 129g/mol.
,• we will also consider 83% C and 17% H. as grams percentage of each element.
Step 1:
1. For C
129g sample *83gC / 100g sample = 107.07 g Carbon
2 . For H
129 g sample * 17g H /100 = 21.93 g Hydrogen
Step 2 :
( Now use the molar mass of each element to determine how many moles of each are present in one mole of a compound )
1. For C
n =mass C/Mol. Mass C
= 107.07g /12.011g/mole
= 8.914 moles
≈ 9 moles
2. For H
n = m/M
= 21.93g /1.00794g/mol
= 21.75
≈ 22 moles
Finally, This means that our compound contain 9 molecules of Carbon and 22 molecules of hydrogen .
Therefore the molecular formula will be C9H22 , 2-Methylpropane .