A 0.472 g sample of an alloy of tin and bismuth is dissolved in sulfuric acid to produce tin(II) and — Stoichiometry / Redox Titration Chemistry Question
Problem Context
A 0.472 g sample of an alloy of tin and bismuth is dissolved in sulfuric acid to produce tin(II) and bismuth(III) ions. This solution is diluted to the mark in a 100 mL volumetric flask and 25.00 mL aliquots are titrated with a 0.0107 M solution of KMnO4, forming tin(IV) and manganese(II) ions. (The bismuth ions are unaffected during this titration.)
Write a balanced equation for the reaction of the MnO4 - ion with Sn(II) in acid solution.
Model Answer
2MnO4– + 5Sn2+ + 16H+ → 2Mn2+ + 5Sn4+ + 8H2O
If an average titration requires 15.61 mL of the MnO4 - solution, calculate the number of moles of MnO4 - used in an average titration.
Model Answer
0.01561 L × 0.0107 mol·L-1 = 1.669 × 10-4 mol
Determine the percentage of tin in the alloy.
Model Answer
1.669 × 10-4 mol × (5 mol Sn2+ / 2 mol MnO4–) × (118.7 g Sn2+ / 1 mol Sn2+) = 0.04953 g of Sn in 25 mL
%Sn = (4 × 0.04953 g / 0.472 g sample) × 100 = 41.97%
State how the end point of the titration is detected.
Model Answer
The endpoint is shown by the persistence of a faint purple color. (Indicating that there is an excess of MnO4-.)
Describe and explain the effect on the calculated percentage of tin in the alloy if the same volume of MnO4- solution is used with the following differences:
i. During the titration the solution pH increases so that MnO2 is formed rather than Mn(II).
Model Answer
The %Sn that is determined is too high. The MnO4-/Sn2+ ratio is 2:3 when MnO2 is formed. Thus, for a given number of moles of Sn, 2/3 as many moles of MnO4- will be used rather than 2/5. Since the calculations assume MnO4-, the latter ratio is used. So (5/2 × 2/3 = 5/3) of the correct moles of Sn would be calculated.
ii. The solution volume in the volumetric flask was above the mark on the flask but a volume of 100. mL was assumed.
Model Answer
The %Sn that is determined would be too low. Each 25 mL aliquot would contain fewer Sn2+ ions, requiring less MnO4- in the titration.
iii. The sample of the original alloy had an oxide coating on it.
Model Answer
The %Sn that is determined would be too low. The oxide coating leads to fewer Sn2+ ions released into solution per g of sample weighed.