Question

 

a. Write the balanced equation in ACID solution for the reaction below:

Fe2++MnO-4→Fe3++Mn2+

b. To standardize a potassium permanganate solution (purple in appearance), a 0.250 g sample of FAS (iron II ammonium sulfate hexahydrate; molar mass = 342 g/mol) is dissolved in 25.00 mL distilled water, then acidified with sulfuric acid. The solution is then titrated with 35.00 mL potassium permanganate solution from a buret until a pale persistent purple color is attained. Calculate the molarity of the potassium permanganate solution.

c. What would be the effect, if any, on the value of the reported standard molarity of the potassium permanganate if the following errors were made? Explain each of your 3 answers.

1.Some drops of water remained in the buret after cleaning but before the permanganate solution was added to the buret.

2.The student neglected to run some permanganate solution through the tip of the buret before taking the initial reading.

3.The student’s lab partner spilled some FAS after weighing it but before titration.

Answer 1

(a)

5Fe²⁺ + MnO₄^- + 8H⁺ ----------------->   5 Fe³⁺ + Mn²⁺ + 4H₂O

(b)

Mass of FAS = 0.250 g

Molar mass of FAS= 342 g

Volume of water =25.0 mL

Volume of potassium permanganate solution =35.00 mL

MnO₄^- + 5Fe²⁺ + 8H⁺ ------------> Mn²⁺ + 5Fe³⁺ + 4H₂O     

By using reaction stoichiometry we find mole ratio between Fe2+ and MnO4-

And mole ratio of Fe2+ : MnO4-   is 5:1

Number of moles of FAS = Fe2+

And number of moles of KMnO4 = MnO4-

moles of FAS = Mass of FAS / Molar mass of FAS

                       = 0.250 g / 342 g per mol

                       = 0.000731 mol FAS

moles of KMnO4 = 0.000731 mol FAS * 1 mol KMnO4 / 5 mol FAS

                            = 0.0001462 mol KMnO4

Molarity of KMnO4 = moles of KMnO4 / Volume in L

The volume is total volume = Volume of water + volume of KMnO4

                                              = 25.00 mL + 35.00 mL = 60.00 mL

= 0.060 L

Molarity of KMnO4 = 0.0001462 mol KMnO4 / 0.060 L

                               = 0.00244 M

Molarity of KMnO4 = 0.00244 M

c)

1) molarity decreases. because volume of permanganate solution increases.

2) molarity decreases

3) molarity increases