5.1.2 (a) Use of the terms Mole Fraction and Partial Pressure
Syllabus
(a) use of the terms mole fraction and partial pressure
What does this mean?
Partial Pressures
If you mix Sulphur Dioxide and Oxygen some Sulphur Trioxide will form and the mixture will settle into an equilibrium containing all three gases.
SO2(g) + O2(g) ⇌ SO3(g)
If you then measured the pressure in the reaction vessel you would be combining the pressure exerted by the SO2, the O2, and the SO3.
Partial Pressure is the term used for the pressure each gas would exert if it occupied the entire vessel entirely on its own at the same temperature.
So the sum of the partial pressures of all the gases in the vessel should equal the total pressure in the vessel.
p(SO2) + p(O2) + p(SO3) = pTotal
Mole Fraction.
In this mixture of gases there will be a measurable number of moles of SO2, O2, and SO3.
The mole fraction of any gas is simply the number of moles of that gas divided by the total moles of all gases.
Putting the two together.
If the above wasn't already exiting enough for you we can combine the two.
It's possible to work out the partial pressure of a gas if you know how many moles of it you have, the total moles of gas and the total pressure.
P1 =Partial Pressure of Gas 1, X1 = mole fraction of Gas 1, n1 = moles of Gas 1, nt = total moles of Gas, Pt =Total Pressure
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