O-LevelChemistryAcids, Bases & Salts5070/22 · May/June 2016 · Q210 Marks
Hydrogen fluoride, HF, has a simple molecular structure. It is soluble in water.
- (a)Suggest one other physical property of hydrogen fluoride.[1]
- (b)(i)Hydrogen fluoride dissociates in water to form dilute hydrofluoric acid. Write an equation to show the dissociation of hydrogen fluoride.[1]
- (b)(ii)Explain why an acidic solution is formed when hydrogen fluoride dissociates in water.[1]
- (c)Dilute hydrofluoric acid reacts with aqueous calcium hydroxide. 2HF(aq) + Ca(OH)2(aq) → CaF2(aq) + 2H2O(l) What is the minimum volume, in cm³, of 0.150 mol/dm³ Ca(OH)2 required to react completely with a solution containing 0.200 g of HF? volume of Ca(OH)2(aq) = .................................................... cm³[3]
- (d)(i)Magnesium reacts with fluorine to make the ionic compound magnesium fluoride. Predict two physical properties of magnesium fluoride. 1. 2.[2]
- (d)(ii)Explain, in terms of electrons, how a magnesium atom reacts with a fluorine molecule, F2, to make a magnesium ion and two fluoride ions.[2]
Mark scheme
Marking points from the official mark scheme (May/June 2016). The mark or mark type is shown in brackets.
- [1] Low melting point/low boiling point
- [1] Does not conduct electricity
- [1] Does not conduct heat
- [1] HF → H+ + F⁻
- [1] Hydrogen ion(s)/H+ present
- [1] Moles of HF = 0.01
- [1] Moles of Ca(OH)2 = 0.005/moles of Ca(OH)2 = 0.5 × moles of HF
- [1] Volume = 33.3 cm³
- [1] High melting point/high boiling point
- [1] Does not conduct electricity as a solid
- [1] Soluble in water
- [1] Conducts electricity as a molten liquid
- [1] Magnesium (atom) loses 2 electrons
- [1] Fluorine (molecule) gains 2 electrons/each fluorine atom gains an electron
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