Question

A hydrocarbon sample with a mass of 6 grams underwent combustion, producing 11 grams of carbon dioxide. if all of the carbon initially present in the compound was converted to carbon dioxide, what was the percent of carbon, by mass, in the hydrocarbon sample?

Answer 1

Let us define first the characteristics of a combustion reaction. It is a reaction that involves reacting a combustible compound with oxygen gas. A compound is combustible if it burns in the presence of oxygen. When the compound is a hydrocarbon, it is made up of C and H atoms, both of which are combustible. You can separate the reactions for C and H.

C + O₂ → CO₂
2 H + 1/2 O₂ → H₂O

So, when a hydrocarbon undergoes combustion, you would expect that the products are always CO₂ and H₂O. Now, let us focus on the reaction for carbon, because we are given with the amount of CO₂ produced. According to the reaction, for every 1 mole Carbon reacted, 1 mole of CO₂ is produced. Knowing that the molar mass of C is 12 g/mol and CO₂ is 44 g/mol,

11 g CO₂ * (1 mol CO₂/44 g) * (1 mol C/1 mol CO₂) * (12 g C/mol)

Amount of C reacted = 3 grams

The percent mass of carbon in the hydrocarbon sample is equal to
Percent mass = (Amount of C reacted/Mass of sample)*100
Percent mass = (3/6)*100
Percent mass = 50%