Glass made with the mineral tellurite, TeO2 (M = 159.6 g mol⁻¹), is often used to manufacture optical fibres - VCE - SSCE Chemistry - Question 5 - 2011 - Paper 1

The oxidation of TeO2 is represented by the half equation:

$TeO_2(s) + 2H^+(aq) + 2e^- → H_2TeO_4(aq)$

The balanced chemical equation will be:

$1TeO_2(s) + 1Cr_2O_7^{2-}(aq) + 8H^+(aq) → 1H_2TeO_4(aq) + 2Cr^{3+} + 7H_2O(l)$

To find the amount of excess dichromate, first calculate the moles of dichromate used:

• Moles of K2Cr2O7 = concentration × volume (L)
• $0.03052 ext{ mol/L} imes 0.05000 ext{ L} = 0.001526 ext{ mol}$

Next, calculate the moles of Fe(NO3)3 reacted:

• Moles of Fe(NO3)3 = concentration × volume (L)
• $0.0525 ext{ mol/L} imes 0.01971 ext{ L} = 0.001034 ext{ mol}$

Using stoichiometry, since 1 mole of Cr2O7²⁻ reacts with 6 moles of Fe³⁺:

• Moles of Cr2O7²⁻ consumed = 1.034 imes rac{1}{6} = 0.0001723 ext{ mol}

Now, calculate excess dichromate:

• Excess moles of Cr2O7²⁻ = initial moles - moles reacted:
• $0.001526 - 0.0001723 = 0.0013537 ext{ mol}$

From the calculated amounts earlier:

• Moles of Cr2O7²⁻ reacted with tellurite:
• $ext{Moles reacted} = 0.001526 - 0.0013537 = 0.0001723 ext{ mol}$

Using the moles of tellurite reacted with the dichromate:

• Based on stoichiometry from the balanced equation:

• Ratio of TeO2 to Cr2O7²⁻ = 1:1 (from the balanced equation)

• Therefore, moles of TeO2 = moles of Cr2O7²⁻ reacted = 0.0001723 mol

Finally, calculate the mass:

• $mass = moles imes molar mass = 0.0001723 ext{ mol} imes 159.6 ext{ g/mol} = 0.0275 ext{ g}$