Question

A balloon contains 8.4 L of helium gas at STP. How many moles of helium are in the balloon?

Answer 1

Answer:

$0.38\ \text{moles of He}$

Explanation:

$\rho$ = Density of helium at stp = 0.179 g/L

M = Molar mass of helium = 4 g/mol

V = Volume of helium = 8.4 L

Mass of helium

$m=\rho V\\\Rightarrow m=0.179\times 8.4\\\Rightarrow m=1.5036\ \text{g}$

Number of moles is given by

$n=\dfrac{m}{M}\\\Rightarrow n=\dfrac{1.5036}{4}\\\Rightarrow n=0.3759\approx0.38\ \text{moles of He}$

The number of moles of helium in the balloon is $0.38\ \text{moles}$