Tungsten trioxide ( WO 3 ) has a rich yellow color and is often used as a pigment in ceramics and paints. To test a ceramic vase for its WO 3 content, a 10.16 g sample of the vase was ground and reduced with Pb ( Hg ) to convert any WO 3 to W 3 + . The resulting W 3 + was transferred to 500.0 mL of 1.00 M HCl . A 100.00 mL aliquot of the HCl solution required 11.15 mL of 0.08026 M potassium permanganate ( KMnO 4 ) to reach the purple endpoint. A blank required 0.11 mL .

Explanation:

Half ionic equation

W3+(aq) --> W6+(aq) +3e-

MnO4-(aq) + 5e- + 8H+(aq) --> Mn2+(aq) + 4H2O(aq)

Balancing for electrons yields:

5W3+ --> 5W6+ + 15e-

3 MnO4- + 15e- + 24 H+ --> 3 Mn2+ + 12 H2O

Overall ionic equation:

5W3+(aq) + 3MnO4-(aq) + 24H+ --> 5W6+(aq) + 3Mn2+(aq) + 12H2O(aq)

Number of moles of MnO4- = mass/molar mass

= 0.08026 *(11.15 - 0.11)/ 1000

= 0.00089 moles

Then reacted with 0.00089 * 5/3 moles of W3+ at the eq point

= 0.0015 mol

Molar concentration = number of moles/volume

= 0.0015 * 5

= 0.0075 moles of W3+

Molar mass of WO3 = 231.8382 g/mol

Mass of WO3 in the flask and sample = molar mass * number of moles

= 0.0075 * 231.8382

= 1.74 g

% WO3 in the sample = mass of WO3/mass of sample * 100

= 1.74/10.16 * 100

= 17.11 %

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