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May 2025 Section II

0/46 marks

2025 · 7 questions · 40 parts · 46 marks

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Context for question 1

Answer the following questions about magnesium.

Context for 1(A)

An incomplete mass spectrum for magnesium is shown in the diagram.

The percent abundance of magnesium-24 is 79%. The percent abundances of the other two natural isotopes of magnesium, magnesium-25 and magnesium-26, are approximately equal.

1(A)(i)Mass Spectrometry, UV-Vis and Infrared SpectroscopyEasy1 mark

Complete the mass spectrum in part A by drawing thick lines in the appropriate locations to represent the percent abundance of magnesium-25 and magnesium-26.

1(A)(ii)Atoms, Isotopes and Relative Atomic MassEasy1 mark

Describe the difference in atomic structure that accounts for the difference in mass between magnesium-25 and magnesium-26.

Context for 1(B)

A student prepares a

1.85 \times 1 0^{- 3}

M solution of Mg(NO

_{3}

)

_{2}

(aq) in beaker 1 and a

2.80 \times 1 0^{- 4}

M solution of NaOH(aq) in beaker 2, as shown.

Beaker 1 with a particle diagram of Mg2+ ions and beaker 2 with the NaOH(aq) solution

The particle diagram shown represents a magnesium ion, Mg

^{2 +}

, in beaker 1. A sodium ion, Na

^{+}

, in beaker 2 has a weaker attraction to water than the Mg

^{2 +}

does. Explain this phenomenon using Coulomb's law and each of the following.

1(B)(i)Ionic Bonding and Ionic LatticesEasy1 mark

The relative charge of the ions

1(B)(ii)Intermolecular Forces, Structure and Properties of SubstancesEasy1 mark

The relative radii of the ions

1(C)pH, Weak Acids, Buffers and Titration CurvesEasy1 mark

Calculate the pH of the solution in beaker 2.

1(D)Reacting Masses and Percentage YieldEasy1 mark

Beakers 1, 2 and 3: 35.00 mL of Mg(NO3)2 solution, 50.00 mL of NaOH solution and the empty beaker 3

A student combines 35.00 mL of

1.85 \times 1 0^{- 3}

M Mg(NO

_{3}

)

_{2}

(aq) with 50.00 mL of

2.80 \times 1 0^{- 4}

M NaOH(aq), as shown in the diagram. Calculate [Mg

^{2 +}

] after the two solutions are combined but before any reaction takes place. (Assume that volumes are additive.)

Context for 1(E)

The dissolution of magnesium hydroxide is represented by the following equation.

Mg (OH)_{2} (s) ⇌ M g^{2 +} (aq) + 2 O H^{-} (aq)

with

K_{s p} = 5.61 \times 1 0^{- 12}

1(E)(i)Solubility, Solubility Product and Partition CoefficientsEasy1 mark

Write the expression for the solubility product constant,

K_{s p}

1(E)(ii)Solubility, Solubility Product and Partition CoefficientsEasy1 mark

After the two solutions are combined in beaker 3 as described in part D, but before any reaction takes place,

[OH^{-}] = 1.65 \times 1 0^{- 4}

M. Using your answer to part D, calculate the value of the reaction quotient, Q.

1(E)(iii)Solubility, Solubility Product and Partition CoefficientsMedium1 mark

Using the reaction quotient, Q, predict whether a precipitate should form as the mixture in beaker 3 approaches equilibrium. Justify your answer.

1(F)Solubility, Solubility Product and Partition CoefficientsMedium1 mark

In a separate experiment, the student adds HNO

_{3}

(aq) to decrease the pH of a saturated solution containing undissolved Mg (OH)

_{2}

(s). Does the amount of undissolved Mg (OH)

_{2}

(s) increase, decrease, or remain the same as the HNO

_{3}

(aq) is added? Justify your answer.