# How many molecules are there in 77.6 L of helium at STP?

$\left(22.4 \text{L "H)/(1"mol "H)=(77.6"L "H)/(x"mol } H\right)$
$x = 3.46 \text{mol } H$
$3.46 \text{mol "Hxx(6.022xx10^(23)"molecules "H)/(1"mol "H)=2.09xx10^(24)"molecules } H$