Identify a conjugate acid–base pair in the equation.

a(i).

The value of the equilibrium constant for the first dissociation at 298 K is 5.01 × 10−4.

State, giving a reason, the strength of citric acid.

a(ii).

The dissociation of citric acid is an endothermic process. State the effect on the hydrogen ion concentration, [H+], and on the equilibrium constant, of increasing the temperature.

a(iii).

Outline one laboratory methods of distinguishing between solutions of citric acid and hydrochloric acid of equal concentration, stating the expected observations.

State the equation for the reaction of each substance with water.

Draw a diagram showing the delocalization of electrons in the conjugate base of butanoic acid.

Deduce the average oxidation state of carbon in butanoic acid.

A 0.250 mol dm−3 aqueous solution of butanoic acid has a concentration of hydrogen ions, [H+], of 0.00192 mol dm−3. Calculate the concentration of hydroxide ions, [OH−], in the solution at 298 K.

Determine the pH of a 0.250 mol dm−3 aqueous solution of ethylamine at 298 K, using section 21 of the data booklet.

Sketch the pH curve for the titration of 25.0 cm3 of ethylamine aqueous solution with 50.0 cm3 of butanoic acid aqueous solution of equal concentration. No calculations are required.

Explain why butanoic acid is a liquid at room temperature while ethylamine is a gas at room temperature.

State a suitable reagent for the reduction of butanoic acid.

Deduce the product of the complete reduction reaction in (e)(i).

Write a balanced equation for the reaction that occurs.

a(i).

Identify a metal, in the same period as magnesium, that does not form a basic oxide.

a(ii).

Calculate the amount of magnesium, in mol, that was used.

b(i).

Determine the percentage uncertainty of the mass of product after heating.

b(ii).

Assume the reaction in (a)(i) is the only one occurring and it goes to completion, but some product has been lost from the crucible. Deduce the percentage yield of magnesium oxide in the crucible.

b(iii).

Evaluate whether this, rather than the loss of product, could explain the yield found in (b)(iii).

c(i).

Suggest an explanation, other than product being lost from the crucible or reacting with nitrogen, that could explain the yield found in (b)(iii).

c(ii).

Calculate coefficients that balance the equation for the following reaction.

d(i).

Ammonia is added to water that contains a few drops of an indicator. Identify an indicator that would change colour. Use sections 21 and 22 of the data booklet.

d(ii).

Determine the oxidation state of nitrogen in Mg3N2 and in NH3.

d(iii).

Deduce, giving reasons, whether the reaction of magnesium nitride with water is an acid–base reaction, a redox reaction, neither or both.

d(iv).

State the number of subatomic particles in this ion.

e(i).

Some nitride ions are 15N3–. State the term that describes the relationship between 14N3– and 15N3–.

e(ii).

The nitride ion and the magnesium ion are isoelectronic (they have the same electron configuration). Determine, giving a reason, which has the greater ionic radius.

e(iii).

Suggest, giving a reason, whether magnesium or nitrogen would have the greater sixth ionization energy.

e(iv).

Suggest two reasons why atoms are no longer regarded as the indivisible units of matter.

State the types of bonding in magnesium, oxygen and magnesium oxide, and how the valence electrons produce these types of bonding.

Outline why the initial reaction should be carried out under a fume hood.

Deduce the equation for the relationship between absorbance and concentration.

Copper(II) ion solutions are blue. Suggest, giving your reason, a suitable wavelength of light for the analysis.

Outline how a solution of 0.0100 mol dm−3 is obtained from a standard 1.000 mol dm−3 copper(II) sulfate solution, including two essential pieces of glassware you would need.

The original piece of brass weighed 0.200 g. The absorbance was 0.32.

Calculate, showing your working, the percentage of copper by mass in the brass.

Deduce the appropriate number of significant figures for your answer in (e)(i).

Comment on the suitability of using brass of this composition for door handles in hospitals.

If you did not obtain an answer to (e)(i), use 70 % but this is not the correct answer.

Suggest another property of brass that makes it suitable for door handles.

Titration is another method for analysing the solution obtained from adding brass to nitric acid.

Copper(II) ions are reduced to copper(I) iodide by the addition of potassium iodide solution, releasing iodine that can be titrated with sodium thiosulfate solution, Na2S2O3(aq). Copper(I) iodide is a white solid.

4I−(aq) + 2Cu2+(aq) → 2CuI (s) + I2(aq)

I2(aq) + 2S2O32−(aq) → 2I−(aq) + S4O62−(aq)

Suggest why the end point of the titration is difficult to determine, even with the addition of starch to turn the remaining free iodine black.