CaHb is an organic compound (gas) that is reacting with O2, v(CaHb)=24l, V(O2)=72l, v(CO2)=48l, V(H2O)=?

Question

Write the balanced equation

find the value of a and b knowing that the mixture is stoichiometryic using the progress table (tableau d'avancement)

Answer 1

For gas phase reactions, Avogadro's principle (n_i ∝ V_i or P_i) applies which means that partial volumes at constant pressure and partial pressures at constant total volume can be used as surrogates for moles, i.e. you can use the stoichiometric coefficient relations to relate partial volumes or partial pressures (whichever applies) rather than moles. I assume that the compound is C_aH_b, so the stoichiometry for complete combustion is:

C_aH_b + cO₂ → aCO₂ + b/2 H₂O (Got the CO₂ and H₂O from the C and H balances respectively)

The O₂ coefficient comes from the O balance where 2c = 2a + b/2 or c = a + b/4

a = V_CO2/V_Org = 48/24 = 2 and c = 2 + b/4 = 72/24 = 3 b = 4

This means C₂H₄ + 3O₂ → 2CO₂ + 2 H₂O

Which checks out.

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