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 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) → Mn²⁺(aq) + 2CO₂(g) + 2H₂O(l) 20.00 mL of the resulting solution is then titrated with a 0.050 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 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.

Photo AI

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

Question 7

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 t... show full transcript

Worked Solution & Example Answer:Question 7 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.

96%

114 rated

Answer

Calculate the initial moles of oxalic acid: $n(H_2C_2O_4) = C \times V = 0.150 \, mol/L \times 0.100 \, L = 0.0150 \, mol$
Calculate the moles of oxalic acid reacted with I₃⁻: n(I_3^-) = C \times V = 0.050 , mol/L \times 0.022 , L = 0.00110 , mol
The stoichiometry for the reaction shows: $n(H_2C_2O_4)_{reacted} = 2 \times n(I_3^-) = 2 \times 0.00110 = 0.00220 \, mol$
Remaining moles of oxalic acid: $n(H_2C_2O_4)_{remaining} = n(H_2C_2O_4)_{initial} - n(H_2C_2O_4)_{reacted}$ $= 0.0150 - 0.00220 = 0.01280 \, mol$

Step 2

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

99%

104 rated

Answer

The moles of oxalic acid used is: $n(H_2C_2O_4)_{used} = n(H_2C_2O_4)_{initial} - n(H_2C_2O_4)_{remaining}$ $= 0.0150 - 0.01280 = 0.00220 \, mol$

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.

96%

101 rated

Answer

From the reaction, the stoichiometry shows: $1 \, mol \; MnO_2 \; reacts \; with \; 1 \, mol \; H_2C_2O_4$
Therefore, the moles of MnO₂ is equal to the moles of oxalic acid used: $n(MnO_2) = n(H_2C_2O_4)_{used} = 0.00220 \, mol$
Calculate the mass of MnO₂: $mass = n \times molar \; mass = 0.00220 \times 86.9 = 0.19118 \, g$
Calculate the percentage of MnO₂ by mass in pyrolusite: $percentage \; of \; MnO_2 = \left( \frac{mass \; of \; MnO_2}{mass \; of \; pyrolusite} \right) \times 100$ $= \left( \frac{0.19118}{1.25} \right) \times 100 = 15.29 \%$

97% of Students

Report Improved Results

98% of Students

Recommend to friends

100,000+

Students Supported

1 Million+

Questions answered

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

Worked Solution & Example Answer:Question 7 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 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.

Join the SSCE students using SimpleStudy...

Other SSCE Chemistry topics to explore