Question

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

8.4 moles He

2.1 moles He

0.38 moles He

190 moles He

Answer 1

`0.38\ \text{moles of He}`

Step-by-step 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* 8.4\\\Rightarrow m=1.5036\ \text{g}`

Number of moles is given by

`n=(m)/(M)\\\Rightarrow n=(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}`