Hey there!:
Molar mass:
O2 = 31.99 g/mol
CO2 = 44 g/mol
By the stoichiometry of the reaction:
C + O2 = CO2
31.99 g O2 --------------- 44 g CO2
mass O2 ------------------ 7.00 g CO2
mass O2 = ( 7.00 * 31.99 ) / 44
mass O2 = 223.93 / 44
mass O2 = 5.089 g
