Pyrolusite, an ore of manganese, contains manganese in the form of MnO₂. A sample of pyrolusite from a newly discovered deposit is analysed to determine the degree of purity of the deposit. To determine the amount of Mn in the pyrolusite sample, 1.25 g of dried pyrolusite was reacted with 100 mL of 0.150 M oxalic acid (H₂C₂O₄). The oxalic acid was in excess, so that all of the MnO₂ reacted according to MnO₂(s) + H₂C₂O₄(aq) + 2H⁺(aq) → Mn²⁺(aq) + 2CO₂(g) + 2H₂O(l) 20.00 mL of the resulting solution is then titrated with a 0.0501 M solution of the triiodide ion I₃⁻(aq) + H₂C₂O₄(aq) → 2CO₂(g) + 2H⁺(aq) + 3I⁻(aq) 22.00 mL of the 0.510 M triiodide solution was needed to react with the remaining oxalic acid. a. Calculate the amount in mole of oxalic acid remaining in the original 100 mL solution after the pyrolusite had been reacted with the oxalic acid. b. Calculate the amount in mole of oxalic acid used to reduce the MnO₂ in the 1.25 g of pyrolusite. c. Calculate the amount in mole of MnO₂ present in the original 1.25 g of pyrolusite and hence the percentage of MnO₂ by mass present in the pyrolusite.

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Pyrolusite, an ore of manganese, contains manganese in the form of MnO₂ - VCE - SSCE Chemistry - Question 7 - 2003 - Paper 1

Question 7

Pyrolusite, an ore of manganese, contains manganese in the form of MnO₂. A sample of pyrolusite from a newly discovered deposit is analysed to determine the degree o... show full transcript

Worked Solution & Example Answer:Pyrolusite, an ore of manganese, contains manganese in the form of MnO₂ - VCE - SSCE Chemistry - Question 7 - 2003 - Paper 1

Step 1

Calculate the amount in mole of oxalic acid remaining in the original 100 mL solution after the pyrolusite had been reacted with the oxalic acid.

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Answer

To find the amount of oxalic acid remaining, we first calculate the initial moles of oxalic acid in the 100 mL solution:

$n(H₂C₂O₄) = 0.150 imes 0.100 = 1.50 imes 10^{-2} \text{ mol}$

Next, we need to calculate the amount of oxalic acid that reacted. The volume of solution used in the titration (20.00 mL) and its concentration (0.0501 M) are used:

$n(I₃⁻) = 0.0501 imes 0.02000 = 1.002 imes 10^{-3} \text{ mol}$

From the stoichiometry of the reaction, 1 mole of I₃⁻ reacts with 1 mole of H₂C₂O₄, so:

$n(H₂C₂O₄) \text{ reacted} = n(I₃⁻) = 1.002 \times 10^{-3} \text{ mol}$

The moles of oxalic acid remaining can thus be calculated as:

$n(H₂C₂O₄) \text{ remaining} = n(H₂C₂O₄)_{initial} - n(H₂C₂O₄)_{reacted}$

$= 1.50 \times 10^{-2} - 1.002 \times 10^{-3} = 1.398 \times 10^{-2} \text{ mol}$

Step 2

Calculate the amount in mole of oxalic acid used to reduce the MnO₂ in the 1.25 g of pyrolusite.

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Answer

We can use the moles of oxalic acid remaining from part (a):

$n(H₂C₂O₄)_{used} = n(H₂C₂O₄)_{initial} - n(H₂C₂O₄)_{remaining}$

Substituting the values, we have:

$n(H₂C₂O₄)_{used} = 1.50 \times 10^{-2} - 1.398 \times 10^{-2} = 1.02 \times 10^{-3} \text{ mol}$

Step 3

Calculate the amount in mole of MnO₂ present in the original 1.25 g of pyrolusite and hence the percentage of MnO₂ by mass present in the pyrolusite.

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Answer

The reaction shows that 1 mole of MnO₂ reacts with 1 mole of H₂C₂O₄. Therefore, the moles of MnO₂ in the 1.25 g of pyrolusite are equal to the moles of oxalic acid used:

$n(MnO₂) = n(H₂C₂O₄)_{used} = 1.02 \times 10^{-3} \text{ mol}$

Now, calculate the mass of MnO₂:

$M(MnO₂) = 86.94 \text{ g/mol}$ $m(MnO₂) = n(MnO₂) \times M(MnO₂) = 1.02 \times 10^{-3} \times 86.94 \approx 0.0889 \text{ g}$

Now, calculate the percentage of MnO₂ by mass in the pyrolusite:

$\text{Percentage} = \frac{m(MnO₂)}{m(pyrolusite)} \times 100 = \frac{0.0889}{1.25} \times 100 \approx 7.112 \%$

Pyrolusite, an ore of manganese, contains manganese in the form of MnO₂ - VCE - SSCE Chemistry - Question 7 - 2003 - Paper 1

Worked Solution & Example Answer:Pyrolusite, an ore of manganese, contains manganese in the form of MnO₂ - VCE - SSCE Chemistry - Question 7 - 2003 - Paper 1

Calculate the amount in mole of oxalic acid remaining in the original 100 mL solution after the pyrolusite had been reacted with the oxalic acid.

Calculate the amount in mole of oxalic acid used to reduce the MnO₂ in the 1.25 g of pyrolusite.

Calculate the amount in mole of MnO₂ present in the original 1.25 g of pyrolusite and hence the percentage of MnO₂ by mass present in the pyrolusite.

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