NJC 2019 H1 Promo Paper 2 solutions

1 NJC Promotional Examination 8873/02/19 [Turn over NJC 2019 H1 Chemistry Promo Paper 2 Solutions 1 (a) A sample of lead contains four stable isotopes with the following percentage abundances. Isotope Percentage abundance / % 204Pb 1.4 206Pb 24.1 207Pb 22.1 208Pb a For Examiner’s Use (i) Define the term relative atomic mass. [1] Relative atomic mass is the weighted average isotopic mass of one atom of an element compared to 1/12 the mass of a 12C atom. (ii) Determine the value of a. Hence calculate the relative atomic mass of lead. Give your answer to two decimal places. [2] % abundance of 208Pb, a = 100 – 1.4 – 24.1 – 22.1 = 52.4% Relative atomic mass of lead = (204 × 1.4%) + (206 × 24.1%) + (207 × 22.1%) + (208 × 52.4%) = 207.24 (2 d.p.) Examiner’s comments: [1] must be 2 dp. ECF from value of a Some students used Pb’s Ar 207.2 from Data Booklet to find the value of a, which is incorrect. For those who gave gmol -1 for relative atomic mas s of lead, they are penalised under units. (iii) Bismuth is on the right side of lead in the Periodic Table. Predict and explain whether bismuth has a higher or lower first ionisation energy compared to lead. [2] Bi has a higher nuclear charge but same screening effect as Pb. Thus there is a stronger nuclear attraction for the most loosely held electrons in Bi, more energy is required to remove the most loosely held electron in Bi. Hence Bi has a higher first IE.

2 NJC Promotional Examination 8873/02/19 [Turn over (b) When the atomic orbitals from two atoms overlap a chemical bond may result. The p orbitals can overlap to form sigma (σ) or pi (π) bonds. When two atoms overlap the z-axis is used to define the internuclear axis. For Examiner’s Use (i) On the diagram below draw two p orbitals (one orbital on each atom) that could overlap to produce a sigma (σ) bond. atom 1 atom 2 [1] (ii) On the diagram below draw two p orbitals (one orbital on each atom) that could overlap to produce a single pi (π) bond. atom 1 atom 2 [1] [Total: 7]

3 NJC Promotional Examination 8873/02/19 [Turn over 2 One means of measuring toxicity is using LD, which stands for "Lethal Dose". LD 50 is the amount of a material which causes the death of 50% of a group of test animals. LD50 value is expressed as the mass of a chemical administered per kg body mass of a test animal. Another means of measuring toxicity is using LC, which stands for "Lethal Concentration". The concentration of the chemical in air that kills 50% of the test animals during th