Considering the definition of STP conditions, 3.35 moles of helium are contained within the balloon.
<h3>Definition of STP condition</h3>The STP conditions refer to the standard temperature and pressure. Pressure values at 1 atmosphere and temperature at 0 ° C are used and are reference values for gases. And in these conditions 1 mole of any gas occupies an approximate volume of 22.4 liters.
<h3>Amount of moles of helium within the balloon</h3>In this case, you know that scientist fills a large, tightly sealed balloon with 75,000 mL of helium at STP.
So, you can apply the following rule of three: if by definition of STP conditions 22.4 liters are occupied by 1 mole of helium, 75 L (75 L= 75000 mL, being 1 L= 1000 L) are occupied by how many moles of helium?
<u><em>amount of moles of helium= 3.35 moles</em></u>
Finally, 3.35 moles of helium are contained within the balloon.
Learn more about STP conditions: