Avagodro's Hypothesis

Avogadro's Law proposed by Amedeo Avogadro

States that under equal conditions of temperature and pressure, equal volumes of gases contain an equal number of molecules.

This hypothesis was not acknowledged in Avogadro's lifetime and it wasn't until Stanislao Cannizzaro, in 1860, demonstrated that it was the solution to the problem of atomic and molecular weights that Avogadro's Law became widely accepted. The number of particles in one mole of a substance was named Avogadro's number in his honour, and is numerically equal to 6.02252 x 10 ²³

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Avogadro’s law, which is derived from this basic idea, says that the volume of a gas maintained at constant temperature and pressure is directly proportional to the number of moles of the gas, or

V = constant x n (where n is the number of moles of the gas)

$P \cdot V = n \cdot R \cdot T$

which can be rearranged thus:

$\frac{V}{n} = \frac{R \cdot T}{P}$

P = the gas absolute pressure, in atm

n = number of moles, in mol

V = the volume, in L

T = the gas absolute temperature, in K

R = the universal gas law constant of 0.0821 L·atm/(mol·K)

At STP (1 atm, 273K), the volume of 1 mole of gas is calculated to be 22.4L.

V=	nRT	=	(1 mol)(0.0821)(273K)	= 22.4L at STP
	P		1 atm

Questions Highlight ot reveal Answers

Determine the volume of 3.0 moles of He at STP.	3.0 moles of He x	22.4L	= 67.2L of He
		1 mole
Determine the volume of 0.5 moles of N₂at STP.	0.5 moles of N₂x	22.4L	=11.2L of N₂
		1 mole
Determine the volume of 10. moles of CO₂at STP.	10. moles of CO₂x	22.4L	=224L of CO₂
		1 mole
Determine the volume of 0.1 moles of Ne at STP.	0.1 moles of Ne x	22.4L	=2.24L of Ne
		1 mole