Answer
The volume occupied by 0.420 mole of $He$ is equal to 9.41 L
Work Step by Step
1. Setup the conversion factors:
For a gas at STP conditions, we can use the equality: $1 \space mole = 22.4 \space L$:
$\frac{1 \space mole}{22.4 \space L}$ and $\frac{22.4 \space L}{1 \space mole}$
2. Calculate the volume of that sample:
$0.420 \space mole \times \frac{22.4 \space L}{1 \space mole} = 9.41 \space L$