Law of combining volumes (Gay-Lussac, 1808) Avogadro law (1811)

Law of combining volumes

(Gay-Lussac, 1808)

"When gases react, the volumes consumed and produced, measured at the same temperature and pressure, are in ratios of small whole numbers".
Consequence. Stoichiometric coefficients in equations of chemical reactions for molecules of gaseous substances shows in which volume combinations gaseous substances are got or react.

examples
a)    
2CO + O2 ® 2CO2

In the time of oxidizing of two volumes of carbon (II) oxide by one volume of oxygen forms 2 volume of carbon (IV) oxide, i.e. the volume of source reaction mixture decrease on 1 volume.
b)     In the time of synthesis of ammonia from elements:

N2 + 3H2 ® 2NH3

One volume of nitrogen is reacting with three volumes of hydrogen: 2 volumes of ammonia are forming – the volume of source reaction mixture will be decreased in 2 times.


Avogadro law
(1811)

Equal volumes of all gases at the same conditions (temperature, pressure) contain the same number of molecules.
This law truth for gaseous substances only.
Consequences:
1.      On mole of any substance in the gaseous state occupies the same volume at the same temperature and pressure.
2.      One mole of any gas in standard conditions (0°C = 273°K, 1 atm = 101.3 kPa) occupies a volume of 22.4 litres.

example 1
What volume of hydrogen at s.t.p. would be evolved at dissolution 4.8 g magnesium in excess of hydrochloric acid?

solution
Mg + 2HCl ® MgCl2 + H2­

At dissolution of 24 g (1 g-mol) magnesium in HCl –– 22.4 l (1 g-mol) of hydrogen is evaluated; at dissolution of 4.8 g of magnesium –– X l of hydrogen.

X =
4.8 * 22.4
————
24
= 4.48 l of hydrogen

example 2
3.17 g of chlorine is borrowing volume, which equal 1 l (at s.t.p.). Calculate the molecular mass of chlorine.

solution
Find mass of 22.4 l chlorine.
1 l
––
3.17 g of hydrogen
22.4 l
––
X g of hydrogen

X = 3.17 * 22.4 = 71 g