How do i find the partial pressure?

[dreamzen]

An atmospheric chemist, studying the pollutant sulfur trioxide (SO₃), places a mixture of sulfur dioxide (SO₂) and oxygen (O₂) in a rigid 4.25 L container at 788 K and 1.85 atm. When the reaction occurs, gaseous sulfur trioxide is formed according to the balanced equation below and the pressure falls to 1.50 atm. 

 

SO₂ + ½O₂ → SO₃

 

Assuming that the temperature at the end of the reaction returns to its original value, what is the partial pressure of sulfur trioxide (SO₃) after the reaction? Provide the answer to one decimal place.

 

[swansont]

What have you done so far?

[chenbeier]

This question was disscussed in an other forum already. The OP did nothing.