IB Chemistry HL Structure 3: Classification of matter questions

Exam-style multiple choice and written questions with full markschemes. Check multiple choice answers straight away, or type, photograph or draw a written answer and get it marked.

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Question 1

Multiple choiceEasy1 mark

A data set gives atomic radii for four consecutive period 3 elements: Na, 186 pm; Mg, 160 pm; Al, 143 pm; Si, 118 pm. What is the mean decrease in atomic radius per step from Na to Si, to the nearest whole picometre?

Show answer and explanation

Answer: B. The mean decrease per step is the total decrease divided by the number of intervals: (186 − 118) pm ÷ 3 = 22.7 pm, which rounds to 23 pm.

A: divides the total decrease by the four elements rather than the three intervals, giving 17 pm. C: divides by two instead of three intervals, giving 34 pm. D: gives the total decrease, 68 pm, rather than the mean decrease per step.

Remember: four consecutive elements have three intervals between them.

Question 2

Paper 2Hard8 marks

Cobalt(II) occurs in differently coloured complexes. A student records the appearance and wavelength of maximum absorption for two solutions.

Complex in solution	Appearance	Wavelength of maximum absorption / nm
[Co(H₂O)₆]²⁺	pink	535
[CoCl₄]²⁻ in chloride-rich solution	blue	610

(a)

State what happens to an electron when visible light is absorbed by a transition-element complex.

[1]

(b)

Use the colour wheel in the data booklet to identify the colour of light absorbed by each solution.

[2]

(c)

A student argues: “Because the solution looks blue, its complex absorbs higher-frequency light than the pink solution.” Assess this claim quantitatively, using the data.

[5]

Question 3

Multiple choiceEasy1 mark

In a compound’s ¹H NMR spectrum, the signal from a CH₂ group is a quartet. Assume the splitting is caused by equivalent hydrogen atoms on adjacent carbon atoms. How many equivalent adjacent hydrogen atoms cause the splitting?

Show answer and explanation

Answer: D. A quartet contains four peaks. When splitting is caused by equivalent hydrogen atoms on adjacent carbon atoms, the number of peaks is one greater than the number of those hydrogen atoms: 4 − 1 = 3.

A: associates no adjacent hydrogen atoms with a quartet; no adjacent hydrogen atoms would give a singlet.
B: associates one adjacent hydrogen atom with a quartet; it gives a doublet.
C: associates two adjacent hydrogen atoms with a quartet; they give a triplet.
Remember: the number of equivalent adjacent hydrogen atoms is one less than the number of peaks.

Question 4

Paper 2Hard9 marks

A laboratory is checking whether molecular formula alone is sufficient to classify oxygen-containing compounds. Figure 1 shows the structural formulae of four compounds.

(a)

Determine the molecular formula of compound A.

[1]

(b)

Using Figure 1, identify two pairs of successive members of homologous series. For each pair, name the series and state the structural change from the smaller member to the larger member.

[4]

(c)

State the general formula for each series identified in (b), using $n$ for the number of carbon atoms.

[2]

(d)

A technician claims that a molecular formula of $\mathrm{C_3H_8O}$ is sufficient to determine uniquely whether a compound belongs to the alcohol or ether series. Evaluate this claim using Figure 1.

[2]

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