Question

How much grams of O2 is required for the complete combustion of 4.0g 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

Answer 2

The required equation is

C +O2 = CO2

Now ,

12g of C reacts with 32g O2

Then, 4g C reacts with 32/12×4 or 10.67g O2

So,

10.67g O2 is required.

Hope it helps.