Review: iron, analysis, equilibrium and pH
Answers are for parents and teachers.
1
4.64 g of Fe₃O₄ (M = 232 g/mol) dissolves in hot concentrated sulphuric acid: Fe₃O₄ + 4H₂SO₄ → FeSO₄ + Fe₂(SO₄)₃ + 4H₂O. Find the masses of the two salts. (FeSO₄ = 152, Fe₂(SO₄)₃ = 400 g/mol) What does the result show about the oxide?
n(Fe₃O₄) = 4.64 ÷ 232 = 0.0200 mol, giving 0.0200 mol of each salt: FeSO₄ 3.04 g and Fe₂(SO₄)₃ 8.00 g. Both iron(II) and iron(III) salts form, which shows that Fe₃O₄ is a mixed oxide, FeO·Fe₂O₃.
2
A 2.50 g sample of impure Na₂SO₄ gives 2.33 g of BaSO₄ with excess BaCl₂ solution. Find the percentage of Na₂SO₄. (BaSO₄ = 233, Na₂SO₄ = 142 g/mol)
n(BaSO₄) = 2.33 ÷ 233 = 0.0100 mol = n(Na₂SO₄). m = 0.0100 × 142 = 1.42 g. Percentage = 1.42 ÷ 2.50 × 100 = 56.8 %.
3
20.0 mL of 0.150 mol/L NaOH is neutralised by 25.0 mL of hydrochloric acid. Find the acid concentration.
Ma × 25.0 = 0.150 × 20.0, so Ma = 3.00 ÷ 25.0 = 0.120 mol/L.
4
A solution gives a reddish-brown precipitate with sodium hydroxide that does not dissolve in excess. Name the cation, write the balanced equation for its chloride, and say which ion would instead give a white precipitate that turns green-white in air.
Iron(III): FeCl₃ + 3NaOH → Fe(OH)₃ + 3NaCl. Iron(II) gives the white precipitate that turns green-white in air.
5
An invented salt AB₂ has s = 2.0 × 10⁻⁵ mol/L. Find Ksp. Then find the pH of a strong acid with [H⁺] = 2.0 × 10⁻⁵ mol/L (log 2 = 0.30).
Ksp = 4s³ = 4 × (2.0 × 10⁻⁵)³ = 4 × 8.0 × 10⁻¹⁵ = 3.2 × 10⁻¹⁴. pH = 5 − log 2 = 5 − 0.30 = 4.70.