How to capture the gas produced by combusting alcohols
If you look at the overall reaction, there is actually no gas produced, but there is a consumption of gas:

$\ce{CH3(CH2)_n-OH + \frac{3}{2} (n + 1)O2 -&gt; (n + 1)CO2 +(n + 2)H2O}$

So 1.5 mol of oxygen are consumed, while only 1 mol of carbon dioxide is formed, so the volume will be less if the temperature is maintained.

To perform this experiment, you must collect the carbon dioxide in a solution of alkali and then titrate the solution.
The carbon dioxide will be converted to carbonate:

$\ce{2NaOH + CO2 -&gt; Na2CO3 +H2O}$

A constant flow of air must be put into the system and the gas that comes out must be bubbled with a diffuser through the alkaline solution.
The air flow must be slow, so that all the carbon dioxide is washed out of the gas stream.

In the titration there will be two equivalence points, the first one of the excess of alkali and the second one of the carbon dioxide. 
This one has a much smaller pH change.

Be sure to make a careful blank of the alkali, because it normally has some carbonate that will add to the value of carbon dioxide.