Combustion Analysis
Find empirical formula from combustion data
Method
Burn organic compound; collect CO₂ and H₂O. Calculate moles of C, H, and (if present) O.
moles C = moles CO₂ (1:1 ratio)
moles H = 2 × moles H₂O (2:1 ratio)
moles O = (mass sample - mass C - mass H) / 16
Example
Given: 1.00 g sample produces 2.20 g CO₂ and 0.90 g H₂O.
n(CO₂) = 2.20 / 44 = 0.050 mol → n(C) = 0.050 mol → mass C = 0.050 × 12 = 0.60 g
n(H₂O) = 0.90 / 18 = 0.050 mol → n(H) = 0.100 mol → mass H = 0.100 × 1 = 0.10 g
mass O = 1.00 - 0.60 - 0.10 = 0.30 g → n(O) = 0.30 / 16 = 0.01875 mol
Ratio C:H:O = 0.050 : 0.100 : 0.01875 ÷ 0.01875 = 2.67 : 5.33 : 1 ≈ 8 : 16 : 3
Answer: Empirical formula C₈H₁₆O₃
Tips
- Always convert to moles before finding ratio.
- Divide all by smallest mole value to get simplest ratio.
- Multiply by integers if needed to get whole numbers.