Question

how much gram of O2 is required for complete combustion of 4.0 g carbon

Answer 1

C + O2 → CO2

here you can see that ,
1mole of Carbon completely combused when 1 mole of O2 present in reaction

e.g 12 g Carbon react with 32g O2

1 g Carbon react with 32/12 g

so, 4 g Carbon react with 32/12 × 4 =32/3 g =10.667