12.5 Non-Ideal Solutions
1. a. i. State Raoult’s law.
ii. 3.94 mol of methanol and 2.22 mol of ethanol miscible to form a solution that obeys Raoult’s law.
The vapour pressures of pure methanol and pure ethanol at 323K are respectively 54.1 kPa and
29.6 kPa. Calculate the vapour pressure of solution produced at that temperature.
b. Sketch and label the vapour pressure – composition curves of a mixture which
i. obeys Raoult’s law
ii. shows negative deviation from Raoult’s law
Explain why the solutions show the above behaviour.
2. (a) State Raoult’s law for a mixture of
liquids.
(b) The figure below shows the
boiling point – composition
diagram for hydrogen fluoride and
water.
3. The figure below shows a partly
completed boiling point-composition
diagram (is obtained at a constant
pressure) for the two completely
miscible liquids P and Q.
(a) Given the boiling point of pure P is
1000C and that pure Q is 900C,
complete the diagram.
(b) A liquid mixture consisting of 50% of
P by mass is subjected to fractional
distillation. What
(i) is the initial boiling point of the
mixture?
(ii) is the composition of the vapour in
equilibrium with the liquid
mixture at its initial boiling point?
4. The figure below shows the boiling
point – composition diagram for the
two miscible liquids X and Y.
(a) Label the liquid – vapour curves on
the diagram above.
(b)(i) What is the boiling point of the
mixture containing 0.6 mole
fraction of X?
(ii) What is the composition of
vapour that produced under the
boiling of this mixture?
(i) A solution of composition X is heated
until it reach boiling point. What is
the boiling point of this solution?
(ii) A mixture of liquid P and Q shows the
ideal behaviour. At a constant
temperature T, the vapour pressure of
pure P and Q are 120kPa and 40kPa
respectively, mole fraction of P in
solution is 0.60. Calculate the total
vapour pressure of this liquid
mixture at temperature T. Calculate
also the vapour composition which is
in equilibrium with the liquid mixture
at temperature T.
(c) On the axes below, sketch the vapour
pressure against composition diagram
you would expected to obtain at
constant temperature for mixtures of
P and Q. Label your diagram liquid
and vapour, where appropriate.
(c) What is the composition of the
distillate that collected
initially when the mixtures of the
following compositions is heated
under the fractional distillation?
(i) 0.3 mole fraction of X
Composition of distillate.
(ii) 0.7 mole fraction of X
Composition of distillate.
(iii) 0.4 mole fraction of X
Composition of distillate.
(d) Explain how the interaction
between the molecules X and Y
causes the formation of this boiling
point / composition curve.
(d) Explain why a mixture of P and Q
cannot be completely separated by
simple distillation.
(e) (i) Describe the changes of
enthalpy and volume occurs
when X and Y is mixed.
(ii) What is the mole fraction of X
that will produce the biggest
variation?
5. The figure 1 shows the vapour
pressure / molar composition curve for
a mixture between propanone
(CH3COCH3) and trichloromethane
(CHCl3) at a constant temperature.
(a)(i) Explain why the curve has the
shape shown.
(ii) What is the nature of interaction
between the molecules of these two
liquids?
6. i) By using the graph paper, draw the boiling point – composition
diagram for the mixtures of liquid Q and R by following the given
information below:
The boiling point of pure Q = 780C
The boiling point of pure R = 800C
The boiling point of azeotrope = 680C
Percentage of Q in azeotrope = 35%
ii) In fractionally distilling a mixture consisting 70% of Q and 30% of
R, explain the changes of temperature and composition of mixture
which will be obtained through this distillation.
[8]
(b) Sketch the boiling point – molar
composition curve for the mixtures of
these two liquids.
7. The table below shows the partial vapour pressure of water and
propan-1-ol in equilibrium with its liquid mixture as a function of
this mixture composition at 250C.
Mole fraction of water
Partial vapour pressure / kPa
Water
Propan-1-ol
0.00
0.00
2.91
0.05
0.56
2.77
0.10
1.08
2.59
0.20
1.79
2.37
0.40
2.65
2.07
0.60
2.89
1.89
0.80
2.91
1.81
0.90
2.93
1.76
0.95
3.03
1.44
0.98
3.13
0.67
1.00
3.17
0.00
By using the data above, construct the vapour pressure - composition
diagram for a mixture of water and propan-1-ol, thus determine
(i) the areas where the liquid mixture shows an ideal behaviour.
(ii) the vapour composition in equilibrium with the liquid mixture that
containing 0.9 mole fraction of water.
(iii) the vapour composition in equilibrium with the liquid mixture
which containing 0.9 mole fraction of water if the mixture having
an ideal behaviour for all of the composition.
(c) How would you expect the
temperature to change when
equimolar quantities of these two
liquids are mixed? Suggest a reason
for your answer.