The answer is approximately 3.3 grams, which can be seen by comparing the before and after cases with the flask and balloon on an electronic balance. The scale on the electronic balance reads in grams and tenths, so the original mass is 261.7 g and the mass after the balloon has expanded is 258.4 g.

The volume of the inflated balloon is:

V = (4/3) pi (d**3)/8 ~ 2.6 liters.

So the mass of the displaces air is:

M = dV = (29g/22.4 liters) * 2.6 liters ~ 3.4 grams,

assuming that the density of air is about 29 grams per one mole of 22.4 liters at standard atmospheric temperature and pressure (a weighted average between 28 g/mole for nitrogen and 32 g/mole for oxygen).

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