Thread: Balancing simple equations

Hi,
I wonder if anyone could help me. I'm a complete beginner in chemistry and am currently struggling to get my head around the fundamentals of balancing equations. I have just attempted to balance some equations given to me in an excersise in a book.

One of the equations I had to balance was: C8H18 + O2 = CO2 + H2O
I balanced this as C8H18 + 25 O2 = 8 CO2 + 9 H2O . However the answer in my excersise book was 2 C8H18 + 25 O2 = 16 CO2 + 18 H2O.

Is my answer wrong (because to me it seems balanced and I can't recognise where I made a mistake)? I can see that the latter end of the equation as answered in my exercise book (16 CO2 + 18 H2O) is just a doubled version of the answer I gave. If my answer is wrong I'm very confused about how to go about this correctly.

I apologise for my incompetence with this; I really am a beginner!

I'd be very appreciative of any help you may be able to offer.

Many thanks

You can check your answer by adding up the elements on each side.

You have 25 x 2 = 50 atoms of oxygen on the left and 8 x 2 + 9 x 1 = 25 atoms of oxygen on the right. You have lost 25 atoms of oxygen so your answer can't be right. You always have to end up with the same number of atoms of each element on both sides of the equation.

In the unbalanced equation C8H18 + O2 = CO2 + H2O you have four species. Every carbon atom ends up in a CO2 molecule and every hydrogen ends up in a water molecule, so you know that every carbon atom needs two oxygen atoms and every hydrogen atom needs one half of an oxygen atom. The total number of oxygen atoms then must be 2 x 8 + 1/2 x 18 = 25 so the balanced equation would be C8H18 + 12 1/2 O2 = 8 CO2 + 9 H2O. But we don't like to have half molecules in equations so we double everything to get rid of the fraction and you end up with the answer in your exercise book.