The balanced equation of the combustion of ethane is:
$2C_2H_6+7O_2->4CO_2+6H_2O$
Then number of mole of oxygen that will be needed to completely react with $5.49mol$ of ethane could be calculated as follows:
$?molO_2=5.49 cancel(mol C_2H_6)xx(7molO_2)/(2cancel(molC_2H_6))=19.2molO_2$
The mass of oxygen needed would be:
$?gO_2=19.2 cancel(molO_2)xx(32.0gO_2)/(1cancel(molO_2))=615gO_2$