Answer to Question #141011 in Physical Chemistry for Varylan

Question

10cm3 of 18.40M sulphuric acid was added to distilled water in a 250ml volumetric flask. The flask was stoppered, turned upside down several times and carefully topped to the mark with more distilled water. After 20minutes it was observed that the solution had dropped by 2ml from the mark.
a) How does this solution behave w.r.t. Raoult’s law? Explain your answer. b) State other observations you may note on mixing the two liquids. c) Draw a labelled vapour pressure-composition phase diagram for the solution. d) Calculate the molarity of the solution in the volumetric flask.

Answer 1

Solution.

a)

For ideal solutions, the total volume when mixed is equal to the sum of the volumes of the solutions. For such systems, Raoul's law is observed. For real solutions, the total volume when mixed is not equal to the sum of the volumes of solutions. Such systems are characterized by deviations from Raoul's law: positive and negative. In our case, there are deviations from Raoul's law. Since the volume decreased after 20 minutes, such a solution is characterized by negative deviations.

b)

Positive deviations from Raoul's law are observed when the total volume of two solutions is greater than the sum of the initial solutions and the mixing process occurs with heat absorption. Negative deviations from Raoul's law are observed when the total volume of two solutions is less than the sum of the initial solutions and the mixing process occurs with the release of heat.

c)

d)

$n(H2SO4) = 18.4 \times 0.01 = 0.184 \ mol$

V(solution) = 248 mL = 0.248 L

$C(H2SO4)= \frac{0.184}{0.248} = 0.74 M$

Answer:

a)

For ideal solutions, the total volume when mixed is equal to the sum of the volumes of the solutions. For such systems, Raoul's law is observed. For real solutions, the total volume when mixed is not equal to the sum of the volumes of solutions. Such systems are characterized by deviations from Raoul's law: positive and negative. In our case, there are deviations from Raoul's law. Since the volume decreased after 20 minutes, such a solution is characterized by negative deviations.

b)

Positive deviations from Raoul's law are observed when the total volume of two solutions is greater than the sum of the initial solutions and the mixing process occurs with heat absorption. Negative deviations from Raoul's law are observed when the total volume of two solutions is less than the sum of the initial solutions and the mixing process occurs with the release of heat.

c)

d)

C = 0.74 M