The formula of a gaseous hydrocarbon which requires 6 times of its own volume of O₂ for complete oxidation and produces 4 times its own volume of CO₂ is CₓH y . The value of y is _____________.

Solution

Chemistry ma’am · 4 steps

For gases, volume ratio is mole ratio, so read the coefficients straight off the combustion equation.

1. Volumes as moles. At the same temperature and pressure, equal volumes of gases hold equal numbers of moles (Avogadro's law). So 1 volume of hydrocarbon, 6 volumes of O₂ and 4 volumes of CO₂ means 1 mol, 6 mol and 4 mol.

   CₓH_y + (x + y/4) O₂ → x CO₂ + y/2 H₂O

2. Carbon from CO2. Each carbon atom gives one CO₂, and 4 volumes of CO₂ form. So x = 4.
3. Hydrogen from O2. The oxygen needed is x + y/4 = 6. Putting in x = 4 gives y/4 = 2, so y = 8.
4. So. The hydrocarbon is C₄H₈, and y is 8.

The common slip: 4. Writing the oxygen needed as x + y/2 (counting moles of water instead of moles of O₂) gives 4, not 8. Each H₂O uses only half an O₂.

The idea to keep. Learn CₓH_y needs (x + y/4) O₂: each C takes one O₂, but four H atoms share one O₂ to make two H₂O.

Answer from the NTA final answer key. Solution by PrepWiser.