By the way, with due respect to MNIO, I would also point out that just because something is described in a general chemistry textbook does not mean it is the best (or even a good) way to do things. Experiments in general chemistry lab are designed to show basic concepts with idealized reaction systems. When trying to adapt these types of experiments to more complex reactions, this is what happens. I don't think anyone would do this kind of experiment in a real research laboratory because of some of the issues pointed out. There are much better, more precise ways to measure gas concentrations. This isn't meant to discourage the OP - even with all the problems, you are learning and you can acquire useful information. Just something to keep in mind.
<sigh>... that quote you mentioned was in reference to AWK's comments about "dry gas" calculations. He clearly didn't follow the experiment. Furthermore we teach this experiment in general chemistry to teach concepts
, not to optimize the process for CO2 in vinegar. That comes later in other classes. Furthermore, BlakeB wasn't seeing a low % yield as one would expect for the gaseous products dissolving in the water. Just the opposite. BlakeB was observing a 250% yield and was looking for help sorting it out. And it turned out, BlakeB had a leak which adversely effect his volume measurement, exactly as I wrote several comments ago.
"Is this the best setup for measuring % yield in a seashell + vinegar reaction" is a totally different question than "what is wrong with my apparatus, I'm observing a 250% yield". BlakeB asked about the second question, right?