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J. W. Hill, R. H. Petrucci, T. W. McCreary, & S. S. Perry General Chemistry, 4th edition.

Problem 14.91.

The photograph shows saturated I2(aq) floating on colorless CCl4(l) (left). After the mixture is vigorously shaken and equilibrium is established, most of the iodine has passed from the water layer to the CCl4 layer, which now has a purple color (right). For this equilibrium, I2(aq)I2(in CCl4),

The concentration of saturated I2(aq) is 1.33×10–3 M.

(a) If 25.0 mL of saturated I2(aq) is shaken with 25.0 mL of CCl4(l), how many milligrams of I2 remain in the aqueous layer at equilibrium?

(b) If the 25.0–mL sample of saturated I2(aq) is shaken with 50 mL of CCl4(l) instead of 25.0 mL, would you expect the mass of I2 remaining in the aqueous layer to be the same, more, or less than that calculated in (a)? Explain, but without doing a calculation.

(c) If the aqueous layer at equilibrium in (a) is brought to equilibrium with a second 25.0–mL sample of CCl4(l), would you expect the mass of I2 remaining in the aqueous layer in this second equilibrium to be the same, more, or less than in (b)? Explain, but without doing a calculation.




Answer:

(a) Set up a table of concentrations:
I2(aq)
I2(CCl4)
Initial
1.33×10–3
0
Change
–x
+x
Equilibrium
1.33×10–3–x
x

So the equilibrium concentration of I2 in CCl4 = 1.31×10–3 M

The concentration of I2 remaining in the aqueous layer = 1.33×10–3 – 1.31×10–3 = 2×10–5 M

The number of mg of I2 in the aqueous layer

= 2×10–5 mol/L × 25 mL × 1L/1000 mL × 2(126.9 g/mol) × 1000 mg/g = 0.1 mg

The initial aqueous solution contained 8.44 mg of I2, so about 99% of the iodine was lost from the water phase in the extraction.

(b) Extraction with 50.0 mL of CCl4 would roughly halve the amount of I2 remaining in the aqueous phase. Consider the experiment done in (a) and then adding an additional 25 mL of CCl4: according to LeChatelier's Principle, the dilution of the CCl4 layer would cause the reaction to try to increase the concentration in the CCl4 layer by shifting the reaction to the right, thereby reducing the amount of I2 in the aqueous layer.

(c) Again, consider the experiment in (a). The first extraction with CCl4 reduced the amount of I2 in the aqueous layer by about 99%. By removing the I2 containing CCl4 and adding another 25 mL of pure CCl4, another 99% of the remaining I2 should be removed from the aqueous layer. Thus, after two successive extractions of 25 mL each, the amount of I2 in the aqueous layer is reduced to about 0.01% of the original amount. In the experiment described in (b), using a single extraction of 50 mL, the amount of I2 remaining in the aqueous layer is about 0.5%, i.e. about 50× as great as using two successive extractions.