Given background: a solid unknown is titrated against KMnO4 to determine the percentage by mass of FeSO4.7H2O.
Question 1:
Typically, a solid iron unknown for titration is dried in an oven to remove adsorbed
water before analysis (the unknowns used in this experiment were dried before
dispensing). Explain how the %(w/w) of FeSO4.7H2O determined would be affected if
the unknowns had not been dried and had contained adsorbed water? (in no more than
30 words.)
Question 2:
Explain why 3M sulfuric acid is added to the iron samples before titration? (Your
answer should be in no more than 30 words.)
KMnO4 redox titration
2014-02-23 6:07 am
回答 (2)
2014-02-26 8:59 am
✔ 最佳答案
Q1:Unknown had not been dried:
Weight of salt obtained will be greater, thus the no. of mole of FeSO4.7H2O will be greater than expected, percentage by mass is greater than actual value compare to that in sample.
Had contained adsorbed water
Weight of salt obtained will be smaller, formula mass of FeSO4 is calculated in stead of FeSO4.7H2O as H2O is absorbed. Hence percentage by mass is smaller than actual value compare to that in sample.
Q2:
Best Answer: To ensure the redox reaction can be undergo with KMnO4, because KMnO4 will undergo redox reaction in acidic condition, so to ensure all the reactants are well reacted.
Alternative Answer: Provide acidic condition for reaction to avoid formation of solids of Fe(OH)2 or Fe(OH)3, so as to ensure accuracy of the reaction mixture obtained.
2014-02-28 8:38 am
Sorry, all wrong.
1.
"Not being dried" is same as "had adsorbed water". The answers are contradictory and inconsistent. Plus, calculation of formula mass will not suddenly jump from FeSO4.7H2O to FeSO4 .
2014-02-28 00:41:41 補充：
2.
Permanganate is also oxidizing under neutral or alkaline medium. However, the reaction(s) is not as same as that in acidic medium -- thus would lead to mis-calculation of molarities.
Fe(II) is unstable in neutral medium and will undergo Spontaneous oxidation with oxygen in air, giving Fe(III).
1.
"Not being dried" is same as "had adsorbed water". The answers are contradictory and inconsistent. Plus, calculation of formula mass will not suddenly jump from FeSO4.7H2O to FeSO4 .
2014-02-28 00:41:41 補充：
2.
Permanganate is also oxidizing under neutral or alkaline medium. However, the reaction(s) is not as same as that in acidic medium -- thus would lead to mis-calculation of molarities.
Fe(II) is unstable in neutral medium and will undergo Spontaneous oxidation with oxygen in air, giving Fe(III).
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