Answer: 1.855 x 10^23
At STP (Standard Temperature and Pressure), 1 mole of any gas occupies 22.4 liters of volume.
Therefore, the number of moles of carbon dioxide in the balloon can be calculated as:
moles of CO2 = volume of balloon / 22.4
moles of CO2 = 6.91 / 22.4
moles of CO2 = 0.308
Now, we can use Avogadro's number to convert moles to molecules.
1 mole of any substance contains 6.02 x 10^23 molecules.
Therefore, the number of molecules of CO2 in the balloon is:
number of molecules = moles of CO2 x Avogadro's number
number of molecules = 0.308 x 6.02 x 10^23
number of molecules = 1.855 x 10^23
Therefore, there are approximately 1.855 x 10^23 molecules of carbon dioxide in the balloon at STP.