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 10.13 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 14.12 mL of 0.08601 M potassium permanganate (KMnO4) to reach the purple endpoint. A blank required 0.14 mL. Balance the reaction below and determine the percent WO3 in the ceramic sample. During the redox titration, the permanganate ion, MnO4 is reduced to Mn2 and W3 is oxidized to WO3.

MnO4- + 5e- --> Mn2+

W3+ --> WO2(2+) + 3e-

Explanation / Answer

The balanced reaction between permangnate ions and W+3 ions will be

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

Given: 100.00 mL aliquot of the HCl solution required 14.12 mL of 0.08601 M potassium permanganate (KMnO4)

Moles of MnO4- = molarity X volume

Moles of KMnO4 = Molarity of KMnO4 X volume of KMnO4 - Molarity of KMnO4 X volume for blank

Moles of KMnO4 = 0.08601(14.12-0.14) = 1.202 millimoles

So here three moles of MnO4- will react with five moles of W+3 ions

Moles of W3+ in 100 mL aliquot = 5/3 x moles of KMnO4

= 5/3 x 1.202 millimoles = 2.0033millimoles
therefore

Now the moles in 100mL = 2.0033millimoles

So moles in 500mL = 500 X 2.0033 / 100 = 10.0165 millimoles
Moles = Mass / Molecular weight

so mass of WO3 = Moles X molecular weight = 10.0165 X 231.84 mg = 2.3222 grams
Mass of sample = 10.13 grams

Mass % of WO3 = Mass of WO3 present X 100 / Mass of sample taken
Mass % of WO3 = 2.322X 100/ 10.13 = 22.92%

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.