Question

During the combustion of 5.00 g of octane, C8H18, 239.5 kcal (1002 kJ) is released.

Balanced equation: 2C8H18+25O2→16CO2+18H2O

1)How many kilocalories are released by the combustion of 17.7 g of C8H18 ?

2)How many moles of octane must be burned to release 540.0 kcal ?

3)How much energy (in kJ) is released by the combustion of 1.77 mol of C8H18?

Answer 1

During the combustion of 5.00 g of octane, C8H18, 239.5 kcal (1002 kJ) is released.
1. So 5.00 g releases 239.5 kcal, how much is released by 17.7 g? Just convert. Two ways to do this.
239.5 kcal x (17.7 g/5.00g) = ? kcal or set up a proportion like this.
(239.5 kcal/5.00g) = (x kcal/17.7 g) and solve for x.
2. How many moles C8H18 in 5.00 g? That's mols = g/molar mass = 5.00/114 = 0.0438. Now it's done the same reasoning OR proportion as #1.
0.0438 moles releases 239.5 kcal so how many moles (x that is) will it take to release 540.0 kcal?
3. Same reasoning or proportion.
0.0438 mols C8H18 releases 1,002 kJ so how many kJ is released burning 1.77 moles?
I shall be happy to check your work if you post it.