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 10.19 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 10.13 mL of 0.08840 M potassium permanganate (KMnO4) to reach the purple endpoint. A blank required 0.18 mL. Determine the percent WO3 in the ceramic sample.

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Explanation / Answer

M of Tungsten (W) = 184 g/mol

M of WO3 = 184 + 3x16 = 232 g/mol

So, moles expected = 10.19/232 = 0.04392 mol

W3 expected = 0.04392/3 = 0.01464 mol

In HCl, KMnO4 is reduced from +7 to +2, thus moles of oxidation per mole of KMnO4 = 5.

So, moles of oxidation = (10.13 - 0.18)*5*0.001*0.08840 = 0.0043979

100 mL out of 500 mL contains, 0.01464/5 mol of W3 = 0.002928 mol W3.

So, oxidation of W3 leads to 3xW6+, thus oxidation required = 3x0.002928x6 moles

= 0.052704 moles.

Now, the actual oxidation required is only 0.0043979 mol

So, % of WO3 = 100 x 0.0043979/0.052704 = 8.34%

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