Consider the reaction: c (s) + o2 (g) -> co2 (g) if 0.3 moles of carbon are allowed to react with 0.4 moles of molecular oxygen, how many moles of co2 are formed?

The balanced chemical reaction is:

C (s) + O2 (g) -> CO2

We are given the amounts of the reactants to be used up in the reaction. These will be the starting point of our calculations. 

C = 0.3 moles
O2 = 0.4 moles

Therefore, the limiting reactant is the carbon. We use the given amount for carbon for the calculations.

0.3 mol C ( 1 mol CO2 / 1 mol C ) = 0.3 mol CO2 formed