Question

A mixture of so2(g) and o2(g), formed by the complete decomposition of so3(g), is collected over water at 34°c at a total pressure of 760 mmhg. if the vapor pressure of water is 40 mmhg at 34°c, what is the partial pressure of so2?

Answer 1

The decomposition of so3 to so2 gas and o2 gas can be described in the balanced chemical equation: 

2so3(g) = 2 so2 (g) + 02(g). 

so assuming a complete reaction, the ratio of so2 gas to total products is 2/3 while that of 02 is 1/3. 

Subtracting water's water vapor pressure, 760-40 mm hG = 720 mm Hg.

then the products partial pressures are

so2 = 2/3 * (720) = 480 mm Hg.
o2 = 720-480 = 240 mm Hg.