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A LevelChemistryCIEExam Questions25. Equilibria (AL)25.2 Partition Coefficients

Partition Coefficients (CIE A Level Chemistry)

Exam Questions

1 hour8 questions
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Medium
Hard
1a1 mark

A saturated solution of silver(I) chloride has a solubility of 1.46 x 10-3 mol dm-3.

Write the expression for the solubility product, Ksp, for silver(I) chloride.

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1b
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Using the information and your answer given in part (a), calculate the solubility product, Ksp, and its units for silver chloride. Show your working.

Ksp = .........................

units = ........................

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1c
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A saturated solution of iron(II) hydroxide has a solubility of 5.82 x 10-6 mol dm-3.

i)
Write the expression for the solubility product, Ksp, for iron(II) hydroxide.
 
[1]
 
ii)
Calculate the solubility product and units for iron(II) hydroxide. Show your working
 
Ksp = ...................
 
units = ..................

[3]

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2a
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The general equilibrium expression for the partition coefficient is

Partition coefficient = begin mathsize 14px style fraction numerator open square brackets equilibrium space concentraton space in space mol space dm to the power of negative 3 end exponent space in space organic space layer close square brackets over denominator open square brackets equilibrium space concentration space in space mol space dm to the power of negative 3 end exponent space aqueous space layer close square brackets end fraction end style

100 cm3 of a 0.5 mol dm-3 solution of aqueous ethylamine, CH3CH2NH2, was shaken with 100 cm3 of an organic solvent at 25 °C and left in a separating funnel to allow an equilibrium to established. The equilibrium equation is:

CH3CH2NH2 (aq) rightwards harpoon over leftwards harpoon CH3CH2NH2 (organic solvent)

Only 50 cm3 of the aqueous layer was run off and titrated against 0.50 mol dm-3 dilute hydrochloric acid. 15.0 cm3 of hydrochloric acid was required to reach the end point.

CH3CH2NH2 (aq) + HCl (aq) → CH3CH2NH3+ (aq) + Cl– (aq) 

i)
Calculate the amount, in moles, of ethylamine that has reacted with hydrochloric acid.



moles of ethylamine that reacted = ....................... mol
[1]
 
ii)
Calculate the amount, in moles, of ethylamine in the aqueous layer.



moles of ethylamine in aqueous layer = ....................... mol
[1]
 
iii)
Calculate the amount, in moles, of ethylamine in the organic layer.



moles of ethylamine in organic layer = ....................... mol
[1]
 
iv)
Calculate the concentration, in mol dm-3, of ethylamine in the aqueous and organic layer.





Concentration in aqueous layer = ................... mol dm-3
 
Concentration in organic layer = ..................... mol dm-3
[1]
 
v)
Calculate the partition coefficient, Kpc, of ethylamine between water and the organic solvent.



Kpc = ..................
[1]

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2b2 marks

Use your answer to part (a) to determine whether ethylamine is more soluble in the aqueous or organic solvent. Explain your answer.

Solvent .................................................

Explanation ..............................................................................................

                   ................................................................................................

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2c
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A solution containing 1.00 g of compound A in 100 cm3 of water was shaken with 10 cm3 of ether. 0.60 g of A was transferred to the ether layer.

Calculate the partition coefficient of compound A between ether and water. Show your working.

 

 

 

 

Kpc = ........................

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3a1 mark

The Ksp value for silver bromide, AgBr (s) is 3.3 x 10-13 at 298 K.

Write the equilibrium equation for the changes that happen in a saturated solution of silver bromide in the presence of some solid.

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3b2 marks

A solution of sodium bromide was added to a saturated solution of silver bromide solution. How would the concentration of dissolved silver ions change. Explain your answer.

Change in concentration of silver ions ..................

Explanation ................................................................................

      ................................................................................

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3c
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Calculate the concentration, in mol dm-3, of silver ions at 298 K in a saturated solution of silver bromide. Show your working.





[Ag+ (aq)] = ....................... mol dm-3

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1a
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i)

State what is meant by partition coefficient.

[2]

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, Kpc, of ammonia between trichloromethane and water.

Show your working.

Kpc = ...........................................................

[2]

iii)

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

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

[2]

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1b1 mark

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

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2a3 marks

The feasibility of a chemical reaction depends on the standard Gibbs free energy change, ΔGɵ.
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 change ....................................

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

[1]

ii)
the dissolving of solid potassium chloride in water

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

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

[1]

iii)
the condensing of water from steam

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

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

[1]

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2b
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Magnesium carbonate can be decomposed on heating.

MgCO3 (s) → MgO (s) + CO2 (g)      ΔHɵ = +117 kJ mol–1


Standard entropies are shown in Table 2.1.

Table 2.1

substance MgCO3 (s) MgO (s) CO2 (g)
Sɵ / J K–1 mol–1 +65.7 +26.9 +214

i)
Calculate ΔGɵ for this reaction at 298 K.
 
Show your working. 
 
ΔGɵ = ............................................. kJ mol–1 
 
[3]

ii)
Explain why this reaction is feasible only at high temperatures.
 
[1]

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2c1 mark

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

Table 2.2

  solubility product in water at 298 K, Ksp / mol2 dm–6
MgCO3 1.0 × 10–5
CaCO3 5.0 × 10–9
SrCO3 1.1 × 10–10

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

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2d3 marks
i)
Write an equation to show the equilibrium for the solubility product of MgCO3. Include state symbols.
 
..................................... ⇌ ..................................................
 
[1]
 
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.
 
[2]

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2e
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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

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2f3 marks

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

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3a
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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 (Ksp = 2.00 x 10-5) will precipitate. Show your working.

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3b
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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 Ksp of 4.5 x 10-14. This reduces this concentration of Mn2+ (aq) by the addition of OH– ions. 

i)
State the expression for Ksp 


[1]
ii)
Calculate the concentration of OH– ions. Show your working. 





[OH–] = ...................... mol dm-3
[2]
iii)
Calculate the minimum pH at 298 K required to prevent clothes from being stained.
(Kw = 1.00 × 10–14 mol2  dm–6 (at 298 K (25 °C))


pH = ...................
[2]

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3c
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Consider the following equilibrium containing a saturated solution of AgBr (s)

AgBr (s) rightwards harpoon over leftwards harpoon 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
    .................................................................................................................. 

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3d3 marks

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

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1a1 mark

Calcium phosphate is a sparingly soluble naturally occurring mineral that is a large component of bones and teeth. The Ksp 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.

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1b
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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

[PO43-] = ........................ mol dm-3

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1c3 marks

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

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2a
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Silver chromate, Ag2CrO4, is a chemical compound of silver and hexavalent chromium that is used in photography.

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

Write the Ksp expression for Ag2CrO4


      Ksp = 

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2b
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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 

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2c
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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. 

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