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 11.27 mL of 0.09177 M potassium permanganate (KMnO4) to reach the purple endpoint. A blank required 0.08 mL. Use the reaction 3MnO4- + 5W3+ +4H+---> 3Mn2+ + 5WO22+ + 2H2O in order to determine the percent WO3 in the ceramic sample.

Explanation / Answer

3 MnO4- + 5 W3+ + 4 H+ => 3 Mn2+ + 5 WO22+ + 2 H2O

Moles of MnO4- = (volume - blank volume) x concentration of MnO4-

= (11.27 - 0.08)/1000 x 0.09177 = 0.0010269 mol


Moles of W3+ in 100 mL aliquot = 5/3 x moles of MnO4-

= 5/3 x 0.0010269 = 0.0017115 mol


Moles of WO3 = moles of W3+ in 500 mL aliquot

= 500/100 x 0.0017115 = 0.00855755 mol


Mass of WO3 = moles x molar mass of WO3

= 0.00855755 x 231.84 = 1.984 g


Mass% of WO3 = mass of WO3/mass of sample x 100%

= 1.984/10.29 x 100%

= 19.28%

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