You must burn 1.17 g C to obtain 2.21 L CO₂ at STP.
The balanced chemical equation is
C + O₂ → CO₂.
Step 1. Convert litres of CO₂ to moles of CO₂.
STP is 0 °C and 1 bar. At STP the volume of 1 mol of an ideal gas is 22.71 L.
Moles of CO₂ = 2.21 L CO₂ × (1 mol CO₂/22.71 L CO₂) = 0.097 31 mol CO₂
Step 2. Use the molar ratio of C:CO₂ to convert moles of CO₂ to moles of C
Moles of C = 0.097 31mol CO₂ × (1 mol C/1 mol CO₂) = 0.097 31mol C
Step 3. Use the molar mass of C to calculate the mass of C
Mass of C = 0.097 31mol C × (12.01 g C/1 mol C) = 1.17 g C
It looks as if you are using the old (pre-1982) definition of STP. That definition gives a value of 1.18 g C.
