You first need to find the molar mass of ethane to figure this out.
[tex]C_{2}H_{6}=2(12.01)\ +\ 6(1.01)\\C_{2}H_{6}=30.08\ g/mol[/tex]
You then need to find the mass of carbon by multiplying its molar mass by how many molecules of carbon there are in the compound.
[tex]C_{2}=2(12.01)\\C_{2}=24.02g[/tex]
You then simply divide the mass of carbon by the mass of the entire compound.
[tex]=\frac{C_{2}}{C_{2}H_{6}} \\\\\\=\frac{24.02}{30.08}\\\\=79.8537234\\[/tex]
Multiply this value by 100 to get your percentage.
[tex]=79.8537234\ x\ 100\\[/tex]
= 79.85% (round as you wish)
