# What is the number of moles in 500 L of #He# gas at STP?

##### 1 Answer

#### Explanation:

The important thing to realize here is that you're working under **STP conditions**, which implies that you can use the molar volume of a gas at STP to find how many *moles* of helium will occupy that volume.

Now, the **molar volume of a gas** represents the volume occupied by **one mole** of a gas under some specific conditions for *pressure* and *temperature*.

Starting from the ideal gas law equation

#color(blue)(PV = nRT)#

you can say that the molar volume of gas at a pressure

#V/n = (RT)/P#

Now, **Standard Temperature and Pressure** conditions are defined as a pressure of

#V/n = (0.0821 * (color(red)(cancel(color(black)("atm"))) * "L")/("mol" * color(red)(cancel(color(black)("K")))) * (273.15 + 0)color(red)(cancel(color(black)("K"))))/(100/101.325color(red)(cancel(color(black)("atm"))))#

#V/n = "22.7 L/mol"#

This of course implies that **one mole** of **any** ideal gas will occupy

In your case, the volume of the gas is said to be equal to

#500 color(red)(cancel(color(black)("L"))) * "1 mole He"/(22.7color(red)(cancel(color(black)("L")))) = "22.026 moles He"#

Rounded to one sig fig, the number of sig figs you have for the volume of the gas, the answer will be

#n_(He) = color(green)("20 moles")#

**SIDE NOTE** *Many textbooks and online sources still list STP conditions as a pressure of* *and a temperature of*

*Under these conditions for pressure and temperature, one mole of any ideal gas occupies* *If these are the values for STP given to you by your instructor, make sure to redo the calculations using* *instead of*