IB Chemistry HL Structure 3: Classification of matter questions

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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?

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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

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?

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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.

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