Friday, January 14, 2011

Molar Volume of a Gas at STP (by Bev)

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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