Re: Possible sources of errors of gas collection with displacement of water?

Sorry for the delay in replying - I've been on holiday!
You seem to have considered all the obvious sources of error - if you are 
happy that the temperature is consistent; that the apparatus has no leaks 
and you are not losing gas in any way (I didn't receive your picture), 
then I think the only other error could be that some of the hydrogen 
peroxide has already partially decomposed. If you leave bottles open, or 
use sample from different bottles you may well find they will vary over a 
period of time; it does decompose fairly quickly in a warm room or 
sunlight. (I have also known students to use dirty apparatus and a small 
trace of catalyst left in a flask will mess up the conc of hydrogen 
peroxide!)
I hope that helps,
Kevin