Question

A sample of a saturated hydrocarbon is placed in a 1.0L vessel at 107^oC. The pressure of the sample at these conditions is .40 atm. A stoichiometrically equivalent amount of oxygen is added to the vessel and the mixture is combusted. Following combustion, the pressure in the vessel is 5.19 atm at 107^oC. Assume all of the products are gases. What is the formula of the hydrocarbon? How many mols of oxygen were added to the container?

Answer 1

Solution.

The general equation of saturated hydrocarbon combustion is

C_nH_2n+2+(3n+1)/2 O₂ = nCO₂+ (n+1)H₂O;

As the pressure is proportional to the amount of substance,

Therefore, the formula of the hydrocarbon is C₆H₁₄. A molar mass of this gas is 86.18 g/mol.

The amount of moles can be found using a formula

Converting pressure to SI units.
P = 40530 Pa
Converting volume to SI units.
V = 0.001 m³
Converting temperature to SI units.
T = 380.15 K
R = 8.314 J/(mol*K)
Computing number of moles using the selected equation.
n = 0.0128 moles.

The reaction of combustion is

2 C₆H₁₄ + 19 O₂ = 12 CO₂ + 14 H₂O

Therefore, the amount of oxygen required is 0.0128*19/2 = 0.122 moles.