How do i balance this equation? C8H18 +O2 -> CO2 + H2O

Question

then you have to calculate mass of CO2 produced when 100g octain undergoes combustion.

Answer 1

C₈H₁₈ +O₂ -> CO₂ + H₂O

 

There are 8 C's on the left, so try 8CO₂ on the right...

 

C₈H₁₈ +O₂ -> 8CO₂ + H₂O

 

Now, there are 18 H's on the left, so try 9H₂O  on the right, to get 18 H's:

 

C₈H₁₈ + O₂ -> 8 CO₂ + 9 H₂O

 

There are a total of 25 O's on the right.  So you'd need 12 1/2 O₂'s to balance that.  You can't have halves of anything, so you'll have to multiply everything by 2 to get rid of the fraction:

 

2 C₈H₁₈ + 25 O₂ -> 16 CO₂ + 18 H₂O

 

So now we have to figure out how many moles of octane there are in 100 g.

 

The molecular mass of octane is:

 

8 C = 8 * 12.011 = 96.088

18 H = 18 * 1.008 = 18.144

 

...for a total of 114.232.  So there are 114.232 grams of octane in 1 mole.  Therefore 100 g of octane is 100 / 114.232 = 0.875 moles.

 

According to the equation, combustion of 2 moles of octane will result in 16 moles of CO₂, so that's 8 moles of CO₂ per mole of octane.  Since there were 0.875 moles of octane, that results in 0.875 * 8 = 7 moles of CO₂.

 

Now figure out the molecular mass of CO₂:

 

C = 12.011

2 O = 2 * 15.999 = 31.998

 

...for a total of 44.009.  Since there were 7 moles of CO₂, that's 7 * 44.099 = 308.063 grams.

 

Answer 2

This process is called stoichiometry and it usually follows the dame general process, which you should learn:
1 - balance equation
2 - use molar mass of given item (in this case the octane) to calculate moles of given item
3 - use the proportion of moles of given to moles of item asked about from the equation to find moles asked about.
4 - use molar mass of asked about item to calculate mass of the asked about item.

 

 

The first trick to balancing an equation is to locate a type of atom that exists in only one molecule on each side of the equation.  I look at this and I see that oxygen is in two places on the right, but carbon and hydrogen are in only one place.  You then pick one of the one place items and balance that.  So I need 8CO2 on the right to balance the eight carbons on the left.  I can do the same thing for the hydrogen and then count how many oxygens I need on the left to balance the ones I count on the right after I get the number in front of CO2 and H20.  Before going on, count everything.  Remember that formulae like C8H18 means 8C + 18H, so when I say 8C8H18, I mean 8(8C+18H).

 

To do the second part, you need to calculate the molar mass of C8H18 by adding up the molar masses of all the 8 carbons and 18 hydrogens.  100g will be some fraction of this total, and that is the number of moles. 

 

Remember that chemical equations count the number of moles, so if in the balanced equation there are 8 moles of CO2 for 8 moles of C8H18, then the number of moles of CO2 has to be the same as the number of moles of C8H18 that you started with.

 

Then you go backwards from the number of moles of CO2, via the molar mass of CO2, to get the mass of the resulting CO2.