Chemistry- Law of Combining Volumes and Avogadro’s Hypothesis, Sept. 10, 2008

Today we discussed The Law of Combining Volumes (suggested by French chemist Joseph Louis Gay Lussac). The law states that when gases react, the volume of the reactants and products, when measure at equal temperature and pressure, are always in whole number ratios.

Example: 2 volumes of hydrogen combined with one volume of oxygen will give 2 volumes of water

This law eventually led to Avogadro’s Hypothesis. Avogadro postulated that equal volumes of all ideal gases, at the same temperature and pressure, contain the same number of molecules. Below, we have 3 containers with the same pressure (expressed in atms instead of kPa) and temperature; we can see that 1 mole of each of the 3 different molecules will occupy the same amount of space.

(Image from:

This law could be described mathematically as:

n1/V1 = n2/V2

n= kV

n α V

where n represents the number of moles, V represents volume and k is some constant.

Molar Volume (MV) describes the space occupied by one mole of gas. The unit for molar volume is L/mol.

MV = V/n

Standard Molar Volume: The molar volume of a gas at STP is 22.4 L/mol.

Note about Ideal Gases: ideal gases are a theoretically volumeless and without attraction. In real life, this does not stand true for many gases; however, we will apply the same formulae and laws to real gases for now.

For homework, refer to page 73 of the text and complete # 38 to 43.


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