__Review:__Recall that Avogadro's Hypothesis states that at the same pressure & temperature, equal volumes of different gases have the same number of particles.

Also, gases will change volume (contract or expand) with changes in pressure in temperature

__So what is molar volume??!__The molar volume of a gas is the volume occupied by 1 mole of gas at Standard Temperature & Pressure (STP), which is 0 degrees Celsius and 1 atmosphere of pressure.

**ANY gas**at STP has a volume of 22.4 litres.

Therefore, the converstion factor is

**mol/22.4L**or

**22.4L/mol**

ex. What is the volume at STP occupied by 12.4 mol of NH3 (g)?

12.5 mol x 22.4 L/mol = 278 L (3 sig figs!)

ex. How many moles are in 85.9 L of H2 at STP?

85.9L x mol/22.4 L = 3.83 mol

ex. How many moles are in 201.6dm^3 of HCl (g) at STP?

1 dm^3 = 1 L

--> 201.6 L x mol/22.4L = 9.000 mol

ex. What volume at STP is occupied by 16.5 g of AsH3 (g)?

MM AsH3 = 77.9

g-->mol-->L (it's incredible how grams, a unit of mass, can be converted into litres, a unit of volume!)

16.5 g x mol/77.9 g x 22.4 L/mol = 4.74 L

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