From the periodic table:
AMU Carbon = 12 g/mol
AMU Hydrogen = 1 g/mol
Given:
20.75 g C
4.24 g H
20.75 g × 1mol/12g C = 1.73 mole C
4.25 g × 1mol/1.008g H = 4.22 mol H
Mole ratio's: (divide by smallest mole present)
1.73 mol C / 1.73 mol C = 1 Carbon
4.22 mol H / 1.73 mol C = 2.44 Hydrogen
Since, 2.44 Hydrogen is close to 2.; multiply both by 2 to get a whole number. Also the pairing of 1 carbon + 2 hydrogen doesn't work.
Empirical Formula = [tex] C_{2} H_{5} [/tex]
Since you are given atomic mass of the compound, you can also determine the Molecular formula .
AMU of empirical = 2(12) + 5(1.008) = 29.04 g/mol
58.04 g/mol actual compound / 29.04 g/mol empirical = 2
double everything again to get the actual molecular formula
[tex] C_{4} H_{10} [/tex]
