# Partial pressure quest

We have an empty vessel with volume of 2L. We put 2.42gr of PCl5 (g) and allowed it to partially decompose at 250 Celsius according to:

PCl5 --> PCl3 + Cl2

the two prodcuts are also gases. The total pressure inside the vessel after this partial decomposment is 359 torr. What is the partial pressure of each gas now?

I calculated that 0.0116 mol of PCl5 were inserted to the vessel, and I choose x to be the amount of molls of PCl5 that became the 2 products, so we have x molls of PCl3, x molls of Cl2 and 0.0116-x molls of PCl5 at the end. But I don't understand - if we add up this amount we get 0.0116+x molls at the end, but we started from 0.0116 molls of material in the first place... I don't understand how this makes sense...

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if we add up this amount we get 0.0116+x molls at the end, but we started from 0.0116 molls of material in the first place... I don't understand how this makes sense...
If 1 mole of PCl5 gets completely decomposed into the two products, what is the total number of moles you get?

2... You got me here also.. Still dont get how this all makes sense

2... You got me here also.. Still dont get how this all makes sense
Why doesn't it make sense? Because you start from an amount of 1 moll and get 2 molls... But maybe because these 2 molls are not made up each of exactly what made up the original 1 moll its ok..

Borek
Mentor
Number of atoms - doesn't change. Total mass - doesn't change. But total number of moles of substances can change any way you like (doesn't mean it always does!). Think water synthesis from elements - 2 moles made out of 3 moles. Think carbon combustion to CO2 in oxygen - 1 mole made of 2 moles. Write reaction equations, balance them and you will see.