[kevinkevin]

  I just took a test and I feel confident with an answer I put but for some reason I second guess myself, which bothers me because that shows that I don't understand it as well as I think I do.  The question went like this; if I am doing a reaction with magnesium and HCl in a gas collecting tube and measuring the volume of hydrogen gas produced per given unit of time, and then do the same reaction with the temperature increased, why is it necessary to measure moles per unit of time instead volume per unit of time?  Here was my reason.  I said that the volume of the hydrogen gas will change with temperature.  The same number of moles of hydrogen gas will occupy a different volume based on the temperature, which is why it is better to measure moles instead of volume.  I also said the pressure will be different, as the temperature changes the vapor pressure of water will either increase of decrease, changing the partial pressure of the hydrogen gas and also contributes to the volume, measuring moles of hydrogen produced eliminates this problem.  Thanks for any feed back.     

[ramboacid]

I don't see why that isn't an acceptable response.

I suspect many other people who took your exam would have forgotten to mention the part about the vapor pressure of water.