DETERMINATION oF IRON BY PERMANGA objective: To determine the percent iron in an

ID: 513882 • Letter: D

Question

DETERMINATION oF IRON BY PERMANGA objective: To determine the percent iron in an unknown To become familiar agent Equipment with permanganate as an oxidizing Buret, analytical balance. fritted crucibles. aspirators Key Points: Solution stability ROCEDU Nancro. will need to be dried at 110c for at use. least I hour before Prepare fritted crucible for filtering as follows: solution of sulfuric eat it gently in a dilute Allow to little hydrogen peroxide in it (10 ml ml 150 ml Hzo. cool and then wash with large quantities of water. Prepare a phosphoric acid solution by adding 20 ml of concentrated phosphoric acid to 30 ml of water. CAUTION: It gets HOT!!! Preparation of KMno solution The solution must be prepared fully since improperly prepared solutions will not store well and may lead to incorrect answers. Dissolve approximately 0.85 grams of KMno. in 500 mL of water and heat to boiling. Boil gently for approximately 15 minutes and then allow to cool. Filter the KMnos solution through the filter crucible using suction filtration. Store in a dark bottle and keep out of direct sunlight or away from heat. The purpose of the filter is to remove any reduced Mn that might clog the buret. Standardization of KMno: Weigh approximately 0.12 grams of Narczou into a 400 ml beaker. Add approximately 200 ml of water and 5 ml of concentrated sulfuric acid. ir well. Be NOT to get acid on you during this process. Wait until the entire so then heat the beaker until the solution just begins to boil. Stop the a small Fill a buret with the KMnoa and titrate the sodium At first add onl pink to portion of the KMnoa since the reaction is slow beginning. You should get a you can purpl color which persists for seconds minutes. this pink color is gone then continue the titration at normal the first faint pink reappears. The appearance of the pink should stay for at least 30 seconds and remain after efficient stirring.

Explanation / Answer

KMnO4 standardization

Write down the reaction between oxalate and KMnO4 as given.

2 MnO4- + 5 C2O42- + 16 H+ -------> 2 Mn2+ + 10 CO2 + 8 H2O

As per the balanced stoichiometric equation,

2 moles MnO4- = 5 moles C2O42-

We will need the molar mass of oxalate for the calculations.

Molar mass of Na2C2O42- = (2*23 + 2*12 + 4*16) g mol-1 = 134 g mol-1

Wt. of Na2C2O4 (g)

Moles of Na2C2O4 = (wt/molar mass)

Moles of KMnO4 = (moles of Na2C2O4)*(2 mole KMnO4/5 mole Na2C2O4)

Volume of KMnO4 (L)

M of KMnO4 = (moles of KMnO4)/(volume of KMnO4 in L)

0.1224

9.134*10-4

3.6536*10-4

0.0322

0.0113

0.1260

9.403*10-4

3.7612*10-4

0.0310

0.0121

Mean M = (0.0113 + 0.0121)/2 M = 0.0117 M

SD = 4.0*10-4 M

%RSD = (SD/Mean M)*100 = (4.0*10-4 M)/(0.0117)*100 = 3.42% (ans)

Determination of unknown

Use the balanced stoichiometric reaction

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

As per the stoichiometric equation,

1 mole MnO4- = 5 moles Fe2+

Wt. Fe(NH4)2(SO4)2.6H2O

Volume KMnO4 (L)

Moles of KMnO4 = (volume of KMnO4 in L)*(M of KMnO4)

Moles of Fe2+ = (moles of KMnO4)*(5 mole Fe2+/1 mole KMnO4)

Mass of Fe = (moles of Fe2+)*(atomic mass of Fe in g mol-1) [ At. mass of Fe = 55.845 g mol-1

% Fe = (mass Fe)/(wt. of sample)*100

0.2566 g

0.0058

6.786*10-5

3.393*10-4

0.01895

7.385

0.2453 g

0.0057

6.669*10-5

(use the mean M of KMnO4 from above)

3.3345*10-4

0.01862

7.591

Mean %Fe = [(7.385 + 7.591)/2]% = 7.488% (ans)

SD = 0.103 %Fe

%RSD = SD/Mean = (0.103 %Fe)/(7.488 %Fe)*100 = 1.375% (ans).