Partition Coefficients (Cambridge (CIE) A Level Chemistry): Exam Questions

i) State what is meant by partition coefficient.

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Ammonia is soluble in both water and organic solvents.

An aqueous solution of ammonia is shaken with the immiscible organic solvent trichloromethane. The mixture is left to reach equilibrium.

Samples are taken from each layer and titrated with dilute hydrochloric acid.

• A 25.0 cm³ sample from the trichloromethane layer requires 13.0 cm³ of 0.100 mol dm^–3 HCl to reach the end-point.

• A 10.0 cm³ sample from the aqueous layer requires 12.5 cm³ of 0.100 mol dm^–3 HCl to reach the end-point.

ii) Calculate the partition coefficient, K_pc, of ammonia between trichloromethane and water.

Show your working.

K_pc = ...........................................................

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iii) Butylamine, C₄H₉NH₂, is also soluble in both water and organic solvents.

Suggest how the numerical value of K_pc of butylamine between trichloromethane and water would compare to the value of K_pc calculated in (a)(ii). Explain your answer.

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Butanamide, C₃H₇CONH₂, is much less basic than butylamine. Explain why.

The feasibility of a chemical reaction depends on the standard Gibbs free energy change, ΔG^ɵ. This is dependent on the standard enthalpy and entropy changes, and the temperature.

State and explain whether the following processes will lead to an increase or decrease in entropy.

i) the reaction of magnesium with hydrochloric acid entropy

entropy change ....................................

explanation ..............................................

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ii) the dissolving of solid potassium chloride in water

entropy change ....................................

explanation ...........................................

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iii) the condensing of water from steam

entropy change ....................................

explanation .............................................

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Magnesium carbonate can be decomposed on heating.

MgCO₃ (s) → MgO (s) + CO₂ (g)      ΔH^ɵ = +117 kJ mol^–1

Standard entropies are shown in Table 2.1.

Table 2.1

i) Calculate ΔG^ɵ for this reaction at 298 K.

 Show your working.  

ΔG^ɵ = ............................................. kJ mol^–1 

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ii) Explain why this reaction is feasible only at high temperatures. 

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Table 2.2 lists values of solubility products, K_sp, of some Group 2 carbonates.

Table 2.2

Deduce the trend in the solubility of the Group 2 carbonates down the group. Justify your answer using the data given.

i) Write an equation to show the equilibrium for the solubility product of MgCO₃. Include state symbols.

..................................... ⇌ ..................................................

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 ii) With reference to your equation in (d)(i), suggest what is observed when a few cm³ of concentrated Na₂CO₃ (aq) are added to a saturated solution of MgCO₃. Explain your answer. 

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Use the data in Table 2.2 to calculate the solubility of MgCO₃ in water at 298 K, in g dm^–3.

 Show your working. 

solubility of MgCO₃ = ................................................ g dm^–3

Describe and explain the variation in the thermal stabilities of the carbonates of the Group 2 elements.

This question is about solubility products

A 1.70 x 10^-3 mol dm^-3 solution of calcium nitrate is mixed with an equal volume of 1.50 x 10^-3 mol dm^-3 potassium sulfate.

Predict whether calcium sulfate (K_sp = 2.00 x 10^-5) will precipitate. Show your working.

Water with a manganese ion, Mn²⁺ (aq), concentration above 1.8 x 10^-6 mol dm^-3 will cause clothes to stain when being washed. The manganese can precipitate as Mn(OH)₂ (s) which has a K_sp of 4.5 x 10^-14. This reduces this concentration of Mn²⁺ (aq) by the addition of OH^– ions. 

i) State the expression for K_sp 

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ii) Calculate the concentration of OH^– ions. Show your working. 

[OH^–] = ...................... mol dm^-3

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iii) Calculate the minimum pH at 298 K required to prevent clothes from being stained. (K_w = 1.00 × 10^–14 mol²  dm^–6 (at 298 K (25 °C))

pH = ...................

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Consider the following equilibrium containing a saturated solution of AgBr (s)

AgBr (s)  Ag⁺ (aq) + Br^– (aq)

Explain what will happen to the solubility of AgBr (s) if the following substances are added to the solution.

• AgNO₃ ..................................................................................................................

• NaBr ..................................................................................................................

• Pb(NO₃)₂ .................................................................................................................. 

Explain why more Mg(OH)₂ dissolves when hydrochloric acid is added to a saturated solution of Mg(OH)₂.

Calcium phosphate is a sparingly soluble naturally occurring mineral that is a large component of bones and teeth. The K_sp value at 25 °C for calcium phosphate is 2.07 x 10^-33. 

Write the balanced equilibrium equation for the dissolution reaction of calcium phosphate.

Use your answer to part (a) to calculate the concentration, in mol dm^-3, of the calcium and phosphate ions in solution. Show your working.

[Ca²⁺] = ......................... mol dm^-3

[PO₄^3-] = ........................ mol dm^-3

Use your answer to part (b) to calculate the mass of solute, in grams, in 150 cm³ of solution at 25 °C. Show your working.

mass of solute = ......................... g

Silver chromate, Ag₂CrO₄, is a chemical compound of silver and hexavalent chromium that is used in photography.

It has a solubility product, K_sp, of 9.0 x 10^-12 at 25 °C. 

Write the K_sp expression for Ag₂CrO₄

      K_sp = 

Use your answer to part (a) to calculate the amount, in moles, of Ag₂CrO₄ that will dissolve in 1 dm^-3 of 0.020 mol dm^-3 K₂CrO₄ solution. Show your working. 

moles of Ag₂CrO₄ = ..................... mol 

Use your answer to part (b) to determine whether a precipitate will form in the solution of K₂CrO₄. Show your working and explain your answer.