Tungsten trioxide (WO3) has a rich yellow color and is often used as a pigment i

Question

Tungsten trioxide (WO3) has a rich yellow color and is often used as a pigment in ceramics and paints. In order to test a ceramic vase for its WO3 content, a 10.29 g sample of the vase was ground and reduced with Pb(Hg) to convert any WO3 to W3 . The resulting W3 was transferred to 500.0 mL of 1.00 M HCl. A 100.00 mL aliquot of the HCl solution required 10.61 mL of 0.09288 M potassium permanganate (KMnO4) to reach the purple endpoint. A blank required 0.20 mL. Balance the reaction below and determine the percent WO3 in the ceramic sample.

Explanation / Answer

Balanced equation,

3MnO4- + 5W3+ + 4H+ ---> 3Mn2+ + WO2^2+ + 2H2O

5 moles of W3+ reacts with 5 moles of MnO4-

moles of KMnO4 used = 0.09288 M x 10.61 ml - 0.09288 M x 0.20 ml = 0.967 mmol

so moles of W3+ in aliquot = 0.967 mmol x 5/3 = 1.611 mmol in 100 ml sample

In 500 ml sample = 1.611 x 5 = 8.055 mmol W3+ present

mass of WO3 = 8.055 mmol x 231.84 g/mol/1000 = 1.867 g

percent of WO3 in ceramic sample = 1.867 x 100/10.29 = 18.14%

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