Question

How many moles of helium gas are contained in a 4.0-L flask at STP?

Answer 1

Answer:

0.1784 moles of helium gas are contained in a 4.0-L flask at STP.

Explanation:

At STP. temperature of the helium gas = T = 273.15 K

At STP, pressure of the helium  gas = P = 1 atm

Volume of the helium  gas at STP  = V = 4.0 L

Moles of the helium  gas = n

Using an ideal gas equation :

$PV=nRT$

$n=\frac{PV}{RT}=\frac{1 atm\times 4.0 L}{0.0821 atm L/mol K\times 273.15 K}$

n = 0.1784 mol

0.1784 moles of helium gas are contained in a 4.0-L flask at STP.

Answer 2

1.0 mole of gas at STP occupies 22.4 Litres.
4/0L / 22.4 L/mole = 0.179 moles He.