IB Chemistry HL S2.1 The ionic model questions

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Question 1

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A calcium fluoride crystal has the empirical formula CaF₂. Which description of its ionic lattice is consistent with this formula?

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Answer: B. An ionic empirical formula gives the simplest whole-number ratio of ions in the lattice. In CaF₂, one Ca²⁺ balances the charge of two F⁻ ions, giving a ratio of 1:2.

A: reverses the subscripts, giving a ratio of 2:1.
C: assigns calcium a +1 charge, so the stated ratio is not charge-balanced.
D: uses equal numbers of ions, leaving the charges unbalanced.

Remember: ionic formulas give ratios of ions in a lattice, not discrete molecules.

An investigation compares the ionic solids magnesium oxide, MgO, and calcium oxide, CaO. Both have the same arrangement of ions in their lattices. The ionic radii for ions in this arrangement are shown below.

Ion	Ionic radius / pm
Mg²⁺	72
Ca²⁺	100
O²⁻	140

(a)

Explain why solid CaO does not conduct electricity but molten CaO does.

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(b)

Use the data to predict which oxide has the larger magnitude of lattice enthalpy. Explain your prediction.

[2]

(c)

Hence, identify which oxide is expected to be less volatile. Explain your answer.

[2]

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