Question

Using the following equation for the combustion of octane, calculate the heat associated with the combustion of 100.0 g of octane assuming complete combustion. The molar mass of octane is 114.33 g/mole. The molar mass of oxygen is 31.9988 g/mole.
2 C8H18 + 25 O2 → 16 CO2 + 18 H2O ΔH°rxn = -11018 kJ
A) -535.4 kJ
B) -4819 kJ
C) -602.3 kJ
D) -385.5 kJ
E) -11018 kJ

Answer 1

Answer: B) -4819 kJ

Explanation:

The balanced reaction for combustion of octane is:

$2C_8H_{18}+25O_2\rightarrow 16CO_2+18H_2O$  $\Delta H^0_{rxn}=-11018kJ$

To calculate the number of moles, we use the equation:

$\text{Number of moles}=\frac{\text{Given mass}}{\text{Molar mass}}$

given mass of octane = 100.0 g

Molar mass of octane = 114.33 g/mol

Putting in the values we get:

$\text{Number of moles}=\frac{100.0g}{114.33g/mol}=0.8747moles$

According to stoichiometry:

2 moles of octane give heat = -11018 kJ

Thus 0.8747 moles of octane give =$\frac{-11018}{2}\times 0.8747=-4819kJ$

Thus -4819 kJ of heat is released by 100.0 g of octane assuming complete combustion.

Answer 2

Answer:

The heat of the reaction = -4819 kJ (option B)

Explanation:

Step 1: Data given

Mass of octane = 100.0 grams

The molar mass of octane is 114.33 g/mol

The molar mass of oxygen is 31.9988 g/mol

ΔH°rxn = -11018 kJ

Step 2: The balanced equation

2 C8H18 + 25 O2 → 16 CO2 + 18 H2O  Hrxn = -11018kJ

Step 3: Calculate moles octane

Moles octane = mass octane / molar mass octane

Moles octane = 100.0 grams / 114.33 g/mol

Moles octane = 0.8747 moles

Since the reaction says for 2 moles octane Hrxn = -11018kJ

For 1 mol octane:

2 8H18 + 12.5 O2 → 8 CO2 + 9 H2O      Hrxn = -11018 /2 = -5509 kJ/mol

The heat of the reaction = 0.8747 moles * -5509 kJ/mol

The heat of the reaction = -4819 kJ (option B)