# Find the mass of #"2.0 L"# of helium gas at STP? The molar mass is #"4.002602 g/mol"#.

##### 2 Answers

To do this problem, we can use the ideal gas law, knowing that we are at STP:

#PbarV = RT#

where

For an ideal gas at STP:

#P = "1 bar"# (standard pressure)#barV_"ideal" = ?# #R = "0.083145 L" cdot "bar/mol" cdot "K"# #T = "273.15 K"# (standard temperature)

We can solve for the ideal molar volume

#barV_"ideal" = (RT)/P#

#= (0.083145*273.15)/(1) = color(green)"22.711 L/mol"#

Then, we know that when assuming helium is *ideal*:

#barV_"ideal" = barV_"He"#

#"22.711 L"/"1 mol ideal gas" = "2.0 L"/(n_"He")#

#n_"He" = "0.0881 mol"#

Lastly, converting to grams:

#0.0881 cancel"mol He" xx ("4.002602 g")/(cancel"mol He")#

#color(blue)("= 0.3525 g He")#

The mass of helium present is

#### Explanation:

You will use the ideal gas law in order to determine moles of helium. Then you will multiply the moles of helium times its molar mass to determine the mass of helium in grams.

**Ideal gas law**

**Known/Given**

Molar mass of helium:

**Unknown**

Moles of helium:

Mass of helium in grams

**Equations**

**Solution**

Rearrange the ideal gas law equation to isolate