Question

A propane burning engine is being evaluated and the exhaust gas collected on a dry basis is 10.1% CO2, 1.1% CO, 4.7% O2 and 84.1% N2. During the test, 0.50 kg of fuel is burned and 1.73 m3 of exhaust gas (again dry) was collected at 15°C and 108 kPa. However, after the test someone questions whether it was really propane that was burned. Using the data above, specify what hydrocarbon was burned.

Answer 1

Well in a hydrocarbon mix you will have several compounds and also impurities. If a dry gas is burned it will be a mixture that con go from C1 to C7+ and impurities.

As you're specifying propane the combustion equation is as follows:

C3H8(g) + 5 O2(g) ---> 3 CO2(g) + 4 H2O(g)

So you are seeing that youre no obtaining an hydrocarbon from the combustion.

Let's find an empircal formula given data:

Due data is in percentage we can assume mass could be 1000g = 1kg = 100%

10.1% CO2 + 1.1% CO + 4.7% O2 + 84.1 N2 = 100%. Because 0.5 kg was burned with divided the % by 2

convert the mass to moles

C= 12; O = 16; N = 14

CO2 = 44 g/mol
CO = 28 g/mol
O2 = 32 g/mol
N2 = 28g/mol

divide each from the mass previously obtained:

10.1/44 = 0.2295

1.1/28 = 0.0392

4.7/32 = 0.1468

84.1/28 = 3.0035

now divide each with the smallest found value:

0.2295/0.0392 = 0.58 = 1

0.0392/0.0392 = 1

0.1468/0.0392 = 3.7 = 4

3.0035/0.0392 = 7.6 = 8

(N2)8 (O2)4 (CO)1 (CO2)1

Once the empirical formula is found, the molecular formula for a compound can be determined if the molar mass of the compound is known. Simply calculate the mass of the empirical formula and divide the molar mass of the compound by the mass of the empirical formula to find the ratio between the molecular formula and the empirical formula. Multiply all the atoms (subscripts) by this ratio to find the molecular formula.

I can only guess for stechiometry as I suppose the hydrocarbon is a mixture where is more concentration of methane (CH4) and could be as follows:

CH4 + 4O2 = CO2 + 2H2O

The other data if to calculate volume from ideal gas equation. Hope it helps in your next calculations.