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 10.05 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 13.50 mL of 0.08202 M potassium permanganate (KMnO4) to reach the purple endpoint. A blank required 0.06 mL. Balance the reaction below and determine the percent WO3 in the ceramic sample.

Answer 1

The reaction to be occured when W3 reacts with KMnO4

5 W³⁺     --> 5 W⁶⁺ +    15 e-

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

Let us calculate the moles of MnO4

Volume of KMnO4 used in titration after substracting from blank = 13.50 -0.06 = 13.44 ml

Moles of KMnO4 = 13.44 x 0.08202 / 1000 = 0.00110234 Moles

0.0010654 Moles of MnO4 could have reacted with (0.00110234 x 5 / 3) 0.018372 Moles

0.017757 Moles of W3 presents in 100 ml aliquot hence for 500 ml = 0.018372 x 5 = 0.009186 Moles

0.009186 Moles of W3 which is equal to the moles of WO3 present in the sample

Let us Convert moles of WO3 (moles of WO3 = moles of W3+) to grams.

Molar mass of WO3 is 231.8382 g/mol

0.008878 Moles of WO3 = 0.009186 x 231.8382 = 2.12972 gm

Percentage of WO3 = 2.12972 x 100 / 10.05 = 21.19 %

21.19 % of WO3 presents in 10.05 g sample