The physical properties of ionic substances

2 marks · Easy

The strong electrostatic forces between ions in an ionic lattice result in high melting points for ionic compounds.

Suppose it takes 780 kJ of energy to melt 1 mole of sodium chloride (NaCl). Calculate the energy required to melt 2.5 moles of sodium chloride.

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1950 kJ (±5)

Also accept: 1950 kilojoules

Common mistakes: Reporting the answer as 312 kJ (incorrect multiplication); Forgetting the unit (kJ)

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Marking points

M1 · 1 mark · Multiplies the given energy per mole (780 kJ) by the number of moles (2.5). e.g. 780 × 2.5

A1 · 1 mark · Final correct answer with unit: 1950 kJ. Accept 1950 kilojoules.

Worked solution

Energy needed for 1 mole = 780 kJ

The energy required to melt 2.5 moles of NaCl is 1950 kJ.