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I did a chemistry lab, in which baking soda and vinegar were reacted in a ziplock bag, and I was to calculate the proper quantity of reactants to fill the bag with CO2. The molar quantity of gas was found using the ideal gas law, where volume, temperature, and pressure were all known. Pressure, however, was set to the rooms air pressure, and I was wondering why that would be the case. Wouldn't a plastic bag full of gas have a fairly high pressure? If its pressure were in equilibrium with the air, wouldn't none of the gas flow out when the bag is opened?

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Strictly speaking, the pressure inside must be certainly higher inside due to the restoring forces generated by the stretching of the plastic bag. It is this force that causes the gases to gush out when punctured or opened.

Balancing forces

Bottom line: the assumption of the pressure inside being equal to atmospheric pressure can be considered correct with a considerably large error margin, provided we are talking about a ziploc bag and not the tyre of a tractor.

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