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(1ai)
Mass of KMnO₄ used: 1.58 g
Volume of solution: 500 cm³ = 0.5 dm³
Molar mass of KMnO₄ = 39 + 55 + 4 × 16 = 158g/mol
Moles = 1.58/158 = 0.01
Concentration = 0.1/0.5 = 0.02mol/dm³
(1aii)
Mass of hydrated FeSO₄ salt = 5.65 g
Volume of solution = 250 cm³ = 0.25 dm³
Concentration in g/dm³
= 5.65/0.25 = 22.6g/dm³
(1b)
From titration, 25 cm³ (0.025 dm³) of FeSO₄ uses 24.80 cm³ (0.0248 dm³) of KMnO₄
Equation: MnO₄⁻ + 5Fe²⁺ –> Mn²⁺ + 5Fe³⁺
Moles of KMnO₄ in 24.80 cm³: 0.02 × 24.80 = 0.000496
Moles Fe²⁺ = 5 × 0.000496 = 0.00248
5.65 g in 250 cm³ = 22.6 g/dm³
Molar mass FeSO₄·xH₂O = 22/0.992 = 228g/mol
FeSO₄·xH₂O = 56 + 32 + 4×16 + x(2+16) = 152 + 18x
152 + 18x = 228
x = 228 – 152/18
x = 4.22
x = 4
(1c)
HNO₃ is an oxidizing acid and may oxidize Fe²⁺ or interfere, causing inaccurate titration results. It can also oxidize Mn²⁺ to higher oxides, affecting the outcome.
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(2a)
(2b)
Salt A: Zinc trioxocarbonate(IV), ZnCO₃
Salt B: Calcium chloride, CaCl₂
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(3ai)
No, the titre value would not be affected.
(3aii)
Washing down the splashed acid or base with a small amount of distilled water does not change the titre because distilled water does not react with the solution. It only ensures that all the reacting solution is inside the main bulk so the reaction goes to completion. The number of moles of acid and base remains the same, so the titre is unchanged.
(3bi)
(i) The pH range of the indicator.
(ii) The type of acid–base reaction or the pH of the equivalence point.
(3bii)
Different acid–base combinations have different pH values at the end point. An indicator must change colour within the pH range where the equivalence point occurs. Therefore, the chosen indicator must have a colour change interval that matches the pH jump of the titration.
(3ci)
Concordant titre values are two or more titre values that are very close to each other (usually within 0.1 cm³), indicating consistent and reliable titration results.
(3cii)
The final burette reading is recorded above the initial reading to account for the volume of liquid delivered and to avoid negative values or confusion in volume calculation.
(3ciii)
A burette filled with aqueous NaOH must be washed thoroughly after use to remove any residue that could react with the next solution used or affect accuracy in subsequent titrations.
(3di)
SO₂
(3dii)
HNO₃
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