ALS7A - Section 7/ Phase Equilibrium / Suggested solutions...

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Section 7/ Phase Equilibrium / Suggested solutions / SKH Bishop Baker Secondary School AL Chemistry Section 7 Phase Equilibrium 80 IA Q.2 (a) (iii) 80 IB Q.9 (a) (i) Boiling point of the distillate collected at 108 o C is 106.1 o C . [1] The concentration of HCl (in mol dm -3 ) is 2.05 (accept values between 2 to 2.1) [1] (ii) When 50 cm 3 of 4.5 M hydrochloric acid was distilled, the mixture would first boil at temperature 105.1 o C and give off a vapour of concentration about 1 M . [1] The vapour evolved has not the same as the composition of the liquid, but always contains a lower proportion of HCl than the liquid in the flask at the same temperature. [1] Since the liquid remaining in the flask becomes stronger in HCl, the boiling point of the liquid rises and continues to rise as the distillation proceeds. The lower curve in the diagram shows the variation in the boiling point of the liquid in the flask as distillation progresses. [1] (OR : The composition of the liquid in the flask alters in the ascending direction of the temperature-composition curve.) The final mixture boils at a constant temperature of 108.6 o C and a constant composition of 6.6 mol dm -3 . [1] (iii) The distillate collected at 108.6 o C has a constant composition and can be used for making standard solutions of hydrochloric acid . [1] (b) (i) Fractional distillation is equivalent to a series of consecutive simple distillation , where the condensed vapour from a previous distillation is used as the liquid for the next distillation . [1] In a fractionating column, the temperature falls in passing from the bottom to the top. As vapour passes up the column, it condenses at first in the lower part. [1] When more hot vapour ascends, the condensed liquid is boiled again, giving vapours which condenses higher up the column . [1] (OR : Latent heat of vaporization given up as the vapour condenses helps to raise the temperature to the boiling points of the previously condensed vapour.) Since the vapour and the residue has different compositions and the vapour always richer in the more volatile component(s), one can separate a mixture into its components having different boiling points. [1] 1
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Section 7/ Phase Equilibrium / Suggested solutions / (ii) 1. Separation of fractions of petroleum from crude oil. [1] 2. Fractional distillation of liquid air, etc. [1] 81 IB Q.9 (b) (i) Run off the lower layer [1], and then use a 25.00 cm 3 pipette [1] with a pipette filler . (Or : Use a 25.00 cm 3 pipette with pipette filler (1), and pass the tip of the pipette below the aqueous layer into the lower layer, not allowing the aqueous layer to enter the pipette. (1)) (ii) xNH 3 (aq) + M 2+ (aq) → M(NH 3 ) x 2+ (aq) [ ½ ] Equilibrium shifts to the left during the titration . [ ½ ] (ii) Concentration of free ammonia in the CHCl 3 layer = 12.70/25.00 x 0.05 mol dm - 3 = 0.0254 mol dm - 3 [ ½ ] Concentration of free ammonia in the aqueous layer = 0.0254 mol dm - 3 x 25 = 0.635 mol dm - 3 [1] Total concentration of ammonia in the aqueous layer
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This note was uploaded on 08/17/2011 for the course CHEM 100 taught by Professor Fleet during the Fall '08 term at Oxford University.

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ALS7A - Section 7/ Phase Equilibrium / Suggested solutions...

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