Balloons like the one shown float in air when filled with a gas less dense than air - Leaving Cert Chemistry - Question b - 2019

To find the number of molecules in the balloon, we use the ideal gas law relations. At standard temperature and pressure (s.t.p.), one mole of gas occupies 22.4 litres. Therefore, if the balloon is filled with 22.4 litres, it contains:

Number of moles = [ \text{Number of moles} = \frac{22.4 \text{ litres}}{22.4 \text{ litres/mole}} = 1 \text{ mole} ]

Using Avogadro's number, which is approximately ( 6.022 \times 10^{23} \text{ molecules/mole} ), the number of molecules is:

[ \text{Number of molecules} = 1 \text{ mole} \times 6.022 \times 10^{23} \text{ molecules/mole} = 6.022 \times 10^{23} \text{ molecules} ]

Thus, the answer is ( 6 \times 10^{23} \text{ molecules} ).