1. Avogadro’s Hypothesis

2. Equal volumes of all gases under the same conditions of Temperature and Pressure contain the same number of molecules. So… we can determine the relative weights of the molecules (molecular weights) of gases.

3. Example #1 At STP, 1 liter of O 2 has a mass of 1.43 g and 1 Liter of CO has a mass of 1.25 g. a)Using Avogadro’s hypothesis, 1 L of each contain the same # of molecules b)Therefore 1 molecule of CO must be 1.25/1.43 times O 2. c)If we know O 2 + 32, then 1.25/1.43 X 32 = 28 (the molecular weight of CO) d)If done precisely we can find precise atomic weights. If done crudely we can at least verify molecular formulas.

4. Example #2 a)SiH 3 (empirical formula) has a Density of 2.9 g/L (at STP) b)Compare to O 2 2.9/1.43 X 32 = 65 (approximate molecular weight) c)Then we can take the molecular weight and divide by empirical weight to determine the chemical formula

5. Molar Volume One mole of all ideal gases have a volume of 22.414 L at STP 22.4L/mole Example: 1 mole of H 2 = 6.022 X 10 23 molecules = 2.0 g = 22.4 L at STP