Final answer:
To find the formula of the hydrocarbon, we compare the volume ratios in the balanced chemical equation for combustion. Given the volumes before and after combustion, we deduce that the hydrocarbon formula is C3H6.
Step-by-step explanation:
To determine the formula of the hydrocarbon, we first need to analyze the combustion process. Given that 375 mL of air is 20% oxygen by volume, there are 75 mL of O₂ involved in the reaction. Since the volume of gases after combustion is 330 mL and we assume all of it is due to CO₂, this means the water vapor has condensed into liquid and is not included in the final volume.
The reaction for combustion of a hydrocarbon CxHy can be written as follows:
CxHy + (x + y/4)O₂ → xCO₂ + (y/2)H₂O(l)
Noting that the volume of a gas is proportional to the moles (assuming constant temperature and pressure), and given that all volumes were measured at the same conditions, we can use the volumes directly to determine the stoichiometry:
15 mL CxHy + 75 mL O₂ → 330 mL CO₂ + H₂O(l)
The coefficients in the balanced reaction indicate that the ratio of CO₂ to hydrocarbon is equal to x, the number of carbon atoms in the hydrocarbon. Thus, x equals 330/15 = 22. Since we have that ratio, we can determine y from the amount of oxygen used. The total oxygen used was 75 mL of which 5 x 15 = 75 mL was used to form CO₂. This means that all the oxygen reacted with the hydrocarbon to form CO₂, and therefore, y must be 2 times the number of unused oxygen molecules which is zero in this case. Hence, the hydrocarbon formula is C3H6.