Benzene and toluene form a nearly ideal solution obeying Raoult's law
over the entire range of composition. Composition is usually expressed as a mole fraction but sometimes other units such as mole percent or mass percent are used. An "
" usually indicates the mole fraction of the liquid phase, a "
" is used for the vapor phase, and a "
" for the total composition of all phases together. The amounts in the phases are calculated from the lever rule:
is the moles of liquid phase,
is the moles of vapor phase, and
are the lengths of the levers indicated by dashed lines on the phase diagrams. The "relative amount" on the bar chart is calculated from
Snapshot 1: when the overall composition
is 0.23 and the pressure is 5.0 kPa, only the liquid phase is stable and its composition is 0.23
Snapshot 2: when the overall composition
is 0.83 and the pressure is 5.0 kPa, only the vapor phase is stable and its composition is 0.83
Snapshot 3: when the overall composition
is 0.50 and the pressure is 5.0 kPa, both phases can coexist. The liquid phase has composition
and the vapor phase has composition
. The liquid lever is
and the vapor lever is
. The relative amounts of the phases are then
. There are twice as many moles of vapor as moles of liquid as shown on the bar chart.